Reference signal reception and cqi computation method and wireless communication apparatus

ABSTRACT

A wireless communication base station apparatus which is able to prevent deterioration in the throughput of LTE terminals even when LTE terminals and LTE+ terminals coexist. In this apparatus, based on the mapping pattern of the reference signals used only in LTE+ terminals, a setting unit sets, in each subframe, the resource block groups where the reference signals used only by the LTE+ terminals are mapped. For symbols mapped to the antennas, an mapping unit maps, to all the resource blocks within one frame, cell specific reference signals used for both LTE terminals and LTE+ terminals. For the symbols mapped to the antennas, the mapping unit maps, to the plurality of resource blocks, of which part of the resource block groups is comprised, in the same subframe within one frame, the cell specific reference signals used only for LTE+ terminals, based on the setting results inputted from the setting unit.

TECHNICAL FIELD

The present invention relates to a reference signal mapping method andradio communication base station apparatus.

BACKGROUND ART

3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) asa downlink communication scheme. According to 3GPP-LTE, a radiocommunication base station apparatus (hereinafter abbreviated as “basestation”) transmits reference signals (RSs) using predeterminedcommunication resources and a radio communication terminal apparatus(hereinafter abbreviated as “terminal”) performs channel estimationusing the received reference signals and demodulates data (seenon-patent literature 1). Furthermore, using reference signals, theterminal performs measurement of receiving quality for adaptive MCS(Modulation and channel Coding Scheme) control, for PMI (PrecodingMatrix Indicator) control in MIMO (Multiple-Input Multiple-Output)transmission or for adaptive scheduling. The terminal then feeds backthe obtained PMI and receiving quality information (CQI: Channel QualityIndicator) to the base station.

Furthermore, when the base station is provided with a plurality ofantennas, the base station can perform diversity transmission. Forexample, the base station can realize high-speed transmission bytransmitting a plurality of data streams from a plurality of antennas(MIMO transmission). In order for the terminal to receive the signaltransmitted with diversity without errors, the terminal has to know achannel condition from a plurality of antennas used for transmission bythe base station to the terminal. Therefore, RSs need to be transmittedfrom all antennas provided for the base station without interferencewith each other. To realize this, 3GPP-LTE employs a method oftransmitting RS from each antenna of the base station using timings andcarrier frequencies different from each other in the time domain andfrequency domain.

FIG. 1 shows a configuration of a 4-antenna base station (4Tx basestation) envisioned by 3GPP-LTE and FIG. 2 shows an RS transmissionmethod by the 4Tx base station (see non-patent literature 2). Here, inFIG. 2, the vertical axis (frequency domain) corresponds to a subcarrierunit and the horizontal axis (time domain) corresponds to an OFDM symbolunit. Furthermore, R0, R1, R2 and R3 represent RSs transmitted fromantennas 0, 1, 2 and 3 (first, second, third and fourth antennas)respectively. Furthermore, in FIG. 2, a unit of one block enclosed by athick line frame (six subcarriers in the frequency domain and fourteenOFDM symbols in the time domain) is called “resource block (RB).” Thoughone RB is comprised of 12 subcarriers according to 3GPP-LTE, it isassumed here that the number of subcarriers, of which one RB iscomprised, is six for ease of explanation. Furthermore, a unit of 1subcarrier×1 OFDM symbol, of which one RB is comprised, is called“resources element (RE).” As is clear from FIG. 2, the 4Tx base stationreduces transmission frequencies of RSs (R2 and R3) from antenna 2(third antenna) and antenna 3 (fourth antenna) to minimize overheadinvolved in RS transmission.

The RSs shown in FIG. 2 are common to all terminals in a cell covered bythe base station and are called “cell-specific RSs (cell-specificreference signals).” Furthermore, the base station may also additionallytransmit RSs (terminal-specific RSs (UE specific reference signals))multiplied by a weight specific to each terminal for beam formingtransmission.

As described above, the number of antennas of a base station accordingto 3GPP-LTE is a maximum of four and a 3GPP-LTE-compliant terminaldemodulates data and measures quality of a downlink signal using RSs (R0to R3 shown in FIG. 2) transmitted from a base station (4Tx basestation) provided with a maximum of four antennas.

By contrast, LTE-advanced which is an evolved version of 3GPP-LTE isstudying a base station equipped with a maximum of 8 antennas (8Tx basestation). However, LTE-advanced is also required to provide a3GPP-LTE-compliant base station to enable terminals compliant with onlya 3GPP-LTE base station (4Tx base station) to communicate. In otherwords, LTE-advanced is required to accommodate both terminals compliantwith only a 4Tx base station (hereinafter referred to as “LTEterminals”) and terminals also compliant with an 8Tx base station(hereinafter referred to as “LTE+ terminals”).

CITATION LIST Non-Patent Literature

-   NPL 1-   3GPP TS 36.213 V8.2.0-   (ftp://ftp.3gpp.org/specs/2008-03/Rel-8/36_series/36213-820.zip)-   NPL 2-   3GPP TS 36.211 V8.2.0-   (ftp://ftp.3gpp.org/specs/2008-03/Rel-8/36_series/36211-820.zip)

SUMMARY OF INVENTION Technical Problem

In LTE-advanced, in order for LTE+ terminals to receive adiversity-transmitted signal without errors, the base station has totransmit RSs corresponding to 8 antennas. For example, as shown in FIG.3, R0 to R7, which are RSs corresponding to 8 antennas may be mapped toall RBs. This allows LTE+ terminals to receive the signal withouterrors. Moreover, terminals can obtain CQI and PMI of each antenna insub-frame units, and can thereby improve throughput by means of MIMOtransmission.

However, LTE terminals grasp only mapping positions of RSs (R0 to R3)shown in FIG. 2. That is, LTE terminals do not know the presence of RSsused only for LTE+ terminals—that is, R4 to R7 shown in FIG. 3.Therefore, in REs to which RSs (R4 to R7) used only for LTE+ terminalsare mapped, LTE terminals receive signals recognizing that data signalshave been mapped. Thus, when LTE terminals and LTE+ terminals coexist,LTE terminals may not be able to correctly receive signals. As a result,the error rate characteristics and throughput of LTE terminalsdeteriorate.

It is therefore an object of the present invention to provide areference signal mapping method and radio communication base stationapparatus capable of preventing deterioration in the throughput of LTEterminals even when LTE terminals and LTE+ terminals coexist.

Solution to Problem

The reference signal mapping method of the present invention maps afirst reference signal used for both a first radio communicationterminal apparatus corresponding to a radio communication base stationapparatus provided with N antennas and a second radio communicationterminal apparatus corresponding to a radio communication base stationapparatus provided with more than N antennas to all resource blocks inone frame, and maps a second reference signal used only for the secondradio communication terminal apparatus to a plurality of resourceblocks, of which part of resource block groups is comprised, in the samesub-frame in one frame.

The radio communication base station apparatus of the present inventionis a radio communication base station apparatus that transmits a firstreference signal used for both a first radio communication terminalapparatus corresponding to a radio communication base station apparatusprovided with N antennas and a second radio communication terminalapparatus corresponding to a radio communication base station apparatusprovided with more than N antennas, and a second reference signal usedonly for the second radio communication terminal apparatus, andcomprises a setting section that sets resource blocks to which thesecond reference signal is mapped per sub-frame based on an mappingpattern of the second reference signal and an mapping section that mapsthe first reference signal to all resource blocks in one frame and mapsthe second reference signal to a plurality of resource blocks, of whichpart of resource block groups is comprised, in the same sub-frame in oneframe.

Advantageous Effects of Invention

Even when LTE terminals and LTE+ terminals coexist, the presentinvention can prevent deterioration in the throughput of LTE terminals.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a conventional4Tx base station;

FIG. 2 shows an RS transmission method by a conventional 4Tx basestation;

FIG. 3 shows an RS transmission method by a conventional 8Tx basestation;

FIG. 4 is a block diagram illustrating a configuration of a base stationaccording to Embodiment 1 of the present invention;

FIG. 5 is a block diagram illustrating a configuration of an LTE+terminal according to Embodiment 1 of the present invention;

FIG. 6 shows an RB to which only RSs used for both LTE terminals andLTE+ terminals according to Embodiment 1 of the present invention aremapped;

FIG. 7 shows an RB to which only RSs used for LTE+ terminals accordingto Embodiment 1 of the present invention are mapped;

FIG. 8 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 1);

FIG. 9 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 1);

FIG. 10 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 1);

FIG. 11 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 2);

FIG. 12 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 2);

FIG. 13 shows an RS mapping pattern according to Embodiment 1 of thepresent invention (mapping method 3);

FIG. 14 shows problems associated with Embodiment 3 of the presentinvention;

FIG. 15 shows an RS mapping pattern according to Embodiment 3 of thepresent invention;

FIG. 16 shows problems associated with Embodiment 4 of the presentinvention;

FIG. 17 shows an RS mapping pattern according to Embodiment 4 of thepresent invention;

FIG. 18 shows another RS mapping pattern according to Embodiment 4 ofthe present invention;

FIG. 19 shows an RS mapping pattern according to Embodiment 5 of thepresent invention;

FIG. 20 shows an RS mapping pattern according to Embodiment 6 of thepresent invention;

FIG. 21 shows another RS mapping pattern according to Embodiment 6 ofthe present invention;

FIG. 22 shows a further RS mapping pattern according to Embodiment 6 ofthe present invention;

FIG. 23 shows an RS mapping pattern according to Embodiment 7 of thepresent invention;

FIG. 24 shows an RS mapping pattern according to Embodiment 8 of thepresent invention; and

FIG. 25 shows another RS mapping pattern according to Embodiment 8 ofthe present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In the followingdescriptions, a base station is provided with eight antennas andtransmits transmission data to LTE terminals and LTE+ terminals.Furthermore, one frame is divided into a plurality of sub-frames.Furthermore, a plurality of subcarriers of one sub-frame are dividedinto a plurality of RBs. That is, one RB is comprised of somesubcarriers of one sub-frame.

Embodiment 1

A configuration of base station 100 according to the present embodimentis shown in FIG. 4.

Encoding/modulation section 101 of base station 100 is provided with asmany encoding sections 11 and modulation sections 12 for transmissiondata as N, the number of terminals with which base station 100 cancommunicate. In encoding/modulation section 101, encoding sections 11-1to 11-N perform encoding processing on transmission data of terminals 1to N and modulation sections 12-1 to 12-N perform modulation processingon encoded transmission data and generate data symbols.Encoding/modulation section 101 determines respective coding rates andmodulation schemes (that is, MCS) of encoding sections 11 and modulationsections 12 based on CQI information inputted from decoding sections118-1 to 118-N.

In encoding/modulation section 102, encoding section 13 performsencoding processing on information indicating an mapping pattern ofcell-specific RSs used only for LTE+ terminals (RS mapping information)and modulation section 14 performs modulation processing on the encodedRS mapping information and generates RS mapping information symbols.Here, base station 100 may broadcast the RS mapping information to allLTE+ terminals in a cell covered by base station 100 using a BCH(Broadcast Channel) signal.

Allocation section 103 allocates data symbols and RS mapping informationsymbols to each subcarrier constituting an OFDM symbol according to theCQI information inputted from decoding sections 118-1 to 118-N andoutputs the allocated symbols to mapping section 104.

Mapping section 104 maps the respective symbols inputted from allocationsection 103 to antennas 110-1 to 110-8. Furthermore, mapping section 104selects a precoding vector used for each antenna based on the PMIinformation inputted from decoding sections 118-1 to 118-N. Mappingsection 104 multiplies the symbol mapped to each antenna by the selectedprecoding vector. Mapping section 104 then outputs the symbol mapped toeach antenna to mapping section 106.

Setting section 105 sets RBs to which cell-specific RSs (R4 to R7)transmitted from antennas 110-5 to 110-8 are mapped per sub-frame basedon the RS mapping information. To be more specific, setting section 105sets RBs to which cell-specific RSs are mapped per sub-frame based onthe mapping pattern indicating mapping positions of cell-specific RSs(R4 to R7) used only for LTE+ terminals. Here, according to the mappingpattern used by setting section 105, cell-specific RSs (R0 to R3) usedfor both LTE terminals and LTE+ terminals are mapped to all RBs in oneframe and cell-specific RSs (R4 to R7) used only for LTE+ terminals aremapped to part of RBs in one frame. Setting section 105 outputs thesetting result to mapping section 106.

Mapping section 106 adds cell-specific RSs (R0 to R7) to symbolsinputted from mapping section 104 and mapped to the respective antennas.To be more specific, in the symbols mapped to antennas 110-1 to 110-4,mapping section 106 maps cell-specific RSs (R0 to R3) used for both LTEterminals and LTE+ terminals to all RBs in one frame. On the other hand,in the symbols mapped to antennas 110-5 to 110-8, mapping section 106maps cell-specific RSs (R4 to R7) used only for LTE+ terminals to theset part of RBs based on the setting result inputted from settingsection 105. Furthermore, when transmission data directed to LTE+terminals are allocated to RBs other than RBs indicated in the settingresult inputted from setting section 105, mapping section 106 mapsterminal-specific RSs to RBs. For example, mapping section 106 uses R4to R7 as terminal-specific RSs. Mapping section 106 may also use R4 toR7 multiplied by terminal-specific weights. Mapping section 106 outputsthe symbol sequence, to which the RS is mapped, to IFFT (Inverse FastFourier Transform) sections 107-1 to 107-8.

IFFT sections 107-1 to 107-8, CP (Cyclic Prefix) adding sections 108-1to 108-8 and radio transmitting sections 109-1 to 109-8 are provided inassociation with respective antennas 110-1 to 110-8.

IFFT sections 107-1 to 107-8 perform IFFT on a plurality of subcarriersconstituting RBs to which symbols are allocated and generate OFDMsymbols which are multicarrier signals. IFFT sections 107-1 to 107-8then output the OFDM symbols generated to CP adding sections 108-1 to108-8 respectively.

CP adding sections 108-1 to 108-8 add the same signal as that at therear end of an OFDM symbol to the head of the OFDM symbol as a CP.

Radio transmitting sections 109-1 to 109-8 perform transmissionprocessing such as D/A conversion, amplification and up-conversion onthe OFDM symbols with the CP added and transmit the OFDM symbols fromantennas 110-1 to 110-8 to the respective terminals. That is, basestation 100 transmits a plurality of data streams from antennas 110-1 to110-8.

On the other hand, radio receiving section 111 receives N signalssimultaneously transmitted from a maximum of N terminals via antennas110-1 to 110-8 and performs receiving processing such asdown-conversion, A/D conversion on the signals.

CP removing section 112 removes CPs from the signals after the receivingprocessing.

FFT (Fast Fourier Transform) section 113 performs FFT on the signalswith the CPs removed and obtains terminal-specific signals multiplexedin the frequency domain. Here, each terminal-specific signal includes adata signal of each terminal and control information including CQIinformation and PMI information of each terminal.

Separation section 114 separates the signal of each terminal inputtedfrom FFT section 113 into data signals and control information of eachterminal. Separation section 114 outputs data signals of terminals 1 toN to demodulation sections 115-1 to 115-N respectively and outputscontrol information of terminals 1 to N to demodulation sections 117-1to 117-N respectively.

Base station 100 is provided with as many demodulation sections 115-1 to115-N, decoding sections 116-1 to 116-N, demodulation sections 117-1 to117-N and decoding sections 118-1 to 118-N as N, the number of terminalswith which base station 100 can communicate.

Demodulation sections 115-1 to 115-N perform demodulation processing onthe data signals inputted from separation section 114 and decodingsections 116-1 to 116-N perform decoding processing on the demodulateddata signals. This allows terminal-specific received data to beobtained.

Demodulation sections 117-1 to 117-N perform demodulation processing onthe control information inputted from separation section 114 anddecoding sections 118-1 to 118-N perform decoding processing on thedemodulated control information. Decoding sections 118-1 to 118-N outputCQI information and PMI information of the control information toencoding/modulation section 101, allocation section 103 and mappingsection 104.

Next, terminal 200 (LTE+ terminal) according to the present embodimentwill be described. FIG. 5 shows a configuration of terminal 200according to the present embodiment.

In terminal 200 shown in FIG. 5, radio receiving sections 202-1 to202-8, CP removing sections 203-1 to 203-8, FFT sections 204-1 to 204-8and extraction sections 205-1 to 205-8 are provided in association withrespective antennas 201-1 to 201-8.

Radio receiving sections 202-1 to 202-8 receive OFDM symbols transmittedfrom base station 100 (FIG. 4) via antennas 201-1 to 201-8 and performreceiving processing such as down-conversion, A/D conversion on the OFDMsymbols.

CP removing sections 203-1 to 203-8 remove CPs from the OFDM symbolsafter the receiving processing.

FFT sections 204-1 to 204-8 perform FFT on the OFDM symbols with CPremoved and obtain signals in the frequency domain.

Extraction sections 205-1 to 205-8 extract cell-specific RSs (R0 to R7)and terminal-specific RSs (e.g. R4 to R7 multiplied by terminal-specificweights) from the signals inputted from FFT sections 204-1 to 204-8based on RS mapping information inputted from decoding section 211.Extraction sections 205-1 to 205-8 output cell-specific RSs to channelestimation section 206 and measuring section 212 and outputterminal-specific RSs to channel estimation section 206. Furthermore,extraction sections 205-1 to 205-8 output the signals inputted from FFTsections 204-1 to 204-8 to spatial receiving processing section 207.Terminal 200 may also acquire RS mapping information by receiving a BCHsignal included in the RS mapping information from base station 100.

Channel estimation section 206 performs channel estimation using thecell-specific RSs and terminal-specific RSs inputted from extractionsections 205-1 to 205-8 and outputs the channel estimation result tospatial receiving processing section 207.

Spatial receiving processing section 207 performs spatial receivingprocessing on the signals inputted from extraction sections 205-1 to205-8—that is, the signals received from antennas 201-1 to 201-8—usingthe channel estimation result inputted from channel estimation section206. Spatial receiving processing section 207 then outputs data signalsof the separated data streams to demodulation section 208 and outputs RSmapping information to demodulation section 210.

Demodulation section 208 performs demodulation processing on the datasignals inputted from spatial receiving processing section 207 anddecoding section 209 performs decoding processing on the demodulateddata signals. In this way, received data is obtained.

Demodulation section 210 performs demodulation processing on the RSmapping information inputted from spatial receiving processing section207 and decoding section 211 performs decoding processing on thedemodulated RS mapping information. Decoding section 211 then outputsthe decoded RS mapping information to extraction sections 205-1 to205-8.

On the other hand, measuring section 212 measures CQIs of antennas 201-1to 201-8 and estimates PMIs to obtain good receiving quality usingcell-specific RSs (R0 to R7) inputted from extraction sections 205-1 to205-8. Measuring section 212 outputs CQI information indicating themeasured CQIs and PMI information indicating the estimated PMI toencoding section 215 as control information.

Encoding section 213 performs encoding processing on transmission dataand modulation section 214 performs modulation processing on the encodedtransmission data and generates data symbols. Modulation section 214outputs the data symbols generated to multiplexing section 217.

Encoding section 215 performs encoding processing on the controlinformation including the CQI information and PMI information inputtedfrom measuring section 212 and modulation section 216 performsmodulation processing on the encoded control information and generatescontrol information symbols. Modulation section 216 outputs the controlinformation symbols generated to multiplexing section 217.

Multiplexing section 217 multiplexes the data symbols inputted frommodulation section 214 and the control information symbols inputted frommodulation section 216 and outputs the multiplexed signal to IFFTsection 218.

IFFT section 218 performs IFFT on a plurality of subcarriers to whichthe signals inputted from multiplexing section 217 are allocated andoutputs the signal after the IFFT to CP adding section 219.

CP adding section 219 adds the same signal as that at the rear end ofthe signal inputted from IFFT section 218 to the head of the signal as aCP.

Radio transmitting section 220 performs transmission processing such asD/A conversion, amplification and up-conversion on the signal with CPadded and transmits the signal from antenna 201-1 to base station 100(FIG. 4).

Next, a cell-specific RS mapping method according to the presentembodiment will be described.

In the following descriptions, as shown, for example, in FIG. 8, oneframe is comprised of five sub-frames (sub-frames 0 to 4). Furthermore,a case will be described as an example where a plurality of subcarriersare uniformly divided into four RBs of RB0 to RB3 in one sub-frame.Furthermore, as shown in FIG. 6 and FIG. 7, one RB is comprised of sixsubcarriers×one sub-frame. Furthermore, cell-specific RSs (R0 to R3)used for both LTE terminals and LTE+ terminals are mapped to REs setbeforehand in an RB as shown in FIG. 6 and FIG. 7. Furthermore,cell-specific RSs (R4 to R7) used only for LTE+ terminals are mapped toREs set beforehand in an RB as shown in FIG. 7.

Furthermore, in the following descriptions, as shown in FIG. 8, RBs(FIG. 6) to which four RSs of R0 to R3 are mapped are represented by “4RSs” and RBs (FIG. 7) to which eight RSs of R0 to R7 are mapped arerepresented by “8 RSs.” That is, in FIG. 8, cell-specific RSs (R0 to R3)used for both LTE terminals and LTE+ terminals are mapped to all RBs inone frame, whereas RSs (R4 to R7) used only for LTE+ terminals aremapped only to RBs represented by 8 RSs.

<Mapping Method 1 (FIG. 8)>

The present mapping method maps cell-specific RSs used only for LTE+terminals only to part of RBs in one frame.

Here, if cell-specific RSs used only for LTE+ terminals are fixedlymapped to only limited part of frequency bands in one frame, basestation 100 can allocate data signals of both LTE+ terminals and LTEterminals to only limited frequency bands. For example, in sub-frame 0to sub-frame 4 in one frame, if cell-specific RSs (R4 to R7) used onlyfor LTE+ terminals are fixedly mapped to only RB0 and RB1 among RB0 toRB3, base station 100 can allocate data signals directed to LTEterminals to only RB2 and RB3. That is, if cell-specific RSs used onlyfor LTE+ terminals are fixedly mapped to only limited part of frequencybands in one frame, RBs to which LTE terminals can be allocated arelimited, which causes the frequency scheduling effect to deteriorate.

Thus, the present mapping method maps cell-specific RSs (R4 to R7) usedonly for LTE+ terminals to RBs of different frequency bands inneighboring sub-frames.

To be more specific, as shown in FIG. 8, R4 to R7 are mapped to RB0 insub-frame 0, R4 to R7 are mapped to RB1 in sub-frame 1, R4 to R7 aremapped to RB2 in sub-frame 2, R4 to R7 are mapped to RB3 in sub-frame 3and R4 to R7 are mapped to RB0 in sub-frame 4.

That is, as shown in FIG. 8, setting section 105 (FIG. 4) of basestation 100 sets RB0 in sub-frame 0 and sets RB1 in sub-frame 1 as RBsto which cell-specific RSs (R4 to R7) used only for LTE+ terminals aremapped. The same applies to sub-frames 2 to 4 as well.

As shown in FIG. 7, mapping section 106 maps R4 to R7 to theircorresponding REs in RB0 of sub-frame 0 and maps R4 to R7 to theircorresponding REs in RB1 of sub-frame 1. The same applies to sub-frames2 to 4 as well.

As shown in FIG. 8, R4 to R7 are mapped to only five RBs out of twentyRBs in one frame (“five sub-frames of sub-frames 0 to 4”×“four RBs ofRB0 to 3”). That is, only R0 to R3 that can be received by LTE terminalsare transmitted in fifteen RBs (“4 RSs” shown in FIG. 8) other than someRBs (“8 RSs” shown in FIG. 8) to which R4 to R7 are mapped. Thus, basestation 100 can allocate LTE terminals to RBs (“4 RSs” shown in FIG. 8)other than some RBs (“8 RSs” shown in FIG. 8) to which R4 to R7 aremapped. This prevents LTE terminals from erroneously receiving REs towhich R4 to R7 are mapped as data symbols and can thereby preventdeterioration of error rate characteristics.

Furthermore, as shown in FIG. 8, RBs (“8 RSs” shown in FIG. 8) to whichR4 to R7 are mapped are mapped to RBs of different frequency domains inneighboring sub-frames. To be more specific, as shown in FIG. 8, R4 toR7 are mapped to RB0 in sub-frame 0, while R4 to R7 are mapped to RB1 ina frequency domain different from that of RB0 in sub-frame 1 adjacent tosub-frame 0. Similarly, R4 to R7 are mapped to RB2 in a frequency domaindifferent from that of RB1 in sub-frame 2 adjacent to sub-frame 1. Thesame applies to sub-frames 3 and 4 as well. That is, R4 to R7 are mappedto an RB shifted by one RB in the frequency domain every sub-frame.

Thus, terminal 200 (LTE+ terminal) can perform CQI measurement and PMIestimation using eight cell-specific RSs (R0 to R7) in any one RB of onesub-frame and can update CQI and PMI for all RBs 0 to 3 in fourcontinuous sub-frames. Terminal 200 (LTE+ terminal) feeds back theobtained CQI and PMI to base station 100. Furthermore, base station 100performs adaptive MCS control based on the fed back CQI and furtherMIMO-transmits transmission data using the fed back PMI. Terminal 200(LTE+ terminal) may also feed back the CQI and PMI obtained in eachsub-frame to the base station every sub-frame. Thus, terminal 200 (LTE+terminal) can reduce the amount of feedback per sub-frame and can feedback newer CQI and PMI per RB—that is, accurate CQI and PMI.Furthermore, terminal 200 (LTE+ terminal) may obtain all CQIs and PMIsof RB0 to RB3 and then feed back CQIs and PMIs to the base station at atime.

Here, high-speed transmission (MIMO transmission) using eight antennasof base station 100 is assumed to be performed in a micro cell having asmall cell radius. Thus, high-speed transmission using eight antennas ofbase station 100 supports only LTE+ terminals that move at low speed.Thus, as shown in FIG. 8, even when a long time interval of foursub-frames is required to perform CQI measurement and PMI estimation inall RBs, the fluctuation of channel quality over four sub-frames isslow, and therefore the deterioration in accuracy of CQI measurement andPMI estimation is small. That is, base station 100 can perform adaptiveMCS control and MIMO transmission using CQI and PMI of sufficientaccuracy from terminal 200 (LTE+ terminal), and can thereby improve thethroughput.

Furthermore, when data of terminal 200 (LTE+ terminal) is allocated toRBs (“4 RSs” shown in FIG. 8) to which R4 to R7 are not mapped, basestation 100 maps terminal-specific RSs for data demodulation (R4 to R7multiplied by terminal-specific weights) to RBs to which data has beenallocated and transmits the data. That is, using terminal-specific RSs,base station 100 can allocate data signals directed to LTE+ terminalsnot only to RBs (“8 RSs” shown in FIG. 8) to which R4 to R7 are mappedbut also to any RB0 to 3. Thus, base station 100 has no more schedulerconstraints when allocating LTE+ terminals, and can thereby improvefrequency scheduling effects.

However, RBs whereby terminal-specific RSs are transmitted varydepending on RBs to which base station 100 allocates LTE+ terminals andbase station 100 notifies only RBs allocated to each LTE+ terminal tothe LTE+ terminal. Therefore, each LTE+ terminal knows only theterminal-specific RSs of the RB allocated to the terminal. That is,other LTE+ terminals cannot perform CQI measurement and PMI estimationusing the terminal-specific RSs. However, according to the presentmapping method, the cell-specific RSs are transmitted on any one RBevery sub-frame, and therefore other LTE+ terminals can perform CQImeasurement and PMI estimation without knowing the terminal-specificRSs.

Thus, the present mapping method maps cell-specific RSs used only forLTE+ terminals only in part of a plurality of RBs in one frame. Thisallows the base station to allocate data signals directed to LTEterminals to RBs other than RBs to which cell-specific RSs used only forLTE+ terminals are mapped. Thus, LTE terminals do not erroneouslyreceive cell-specific RSs used only for LTE+ terminals as data signals,and it is thereby possible to prevent deterioration of error ratecharacteristics. Therefore, even when LTE terminals and LTE+ terminalscoexist, the present mapping method can prevent deterioration in thethroughput of LTE terminals. Furthermore, when data signals directed toLTE+ terminals are allocated to RBs to which cell-specific RSs used onlyfor LTE+ terminals are not mapped, the base station mapsterminal-specific RSs to RBs. This allows the base station to allocatedata signals directed to LTE+ terminals to all RBs, and it is therebypossible to improve the frequency scheduling effect.

Furthermore, the present mapping method maps cell-specific RSs used onlyfor LTE+ terminals to RBs of different frequency domains betweenneighboring sub-frames and maps RSs to an RB shifted by one RB everysub-frame. This ensures that LTE+ terminals receive cell-specific RSsover a plurality of continuous sub-frames even in RBs to which datasignals of the LTE+ terminals are not allocated. In this way, the LTE+terminals can perform CQI measurement and PMI estimation accurately inall frequency bands. The amount of shift of cell-specific RSs does notnecessarily have to be one RB.

The present mapping method may also use an RS mapping pattern whose timedomain and frequency domain differ from one cell to another. Forexample, of two neighboring base stations, one base station may use themapping pattern shown in FIG. 8, while the other base station may use anmapping pattern shown in FIG. 9. In the mapping pattern shown in FIG. 8,R4 to R7 are mapped to RBs 0, 1, 2, 3 and 0 in order of sub-frames 0, 1,2, 3 and 4, while in the mapping pattern shown in FIG. 9, R4 to R7 aremapped to RBs 0, 2, 1, 3 and 0 in order of sub-frames 0, 1, 2, 3 and 4.That is, in the mapping pattern shown in FIG. 9, R4 to R7 are mapped tosome RBs shifted by a plurality of RBs (here, two RBs) in the frequencydomain every sub-frame in one frame. Alternatively, while one of the twoneighboring base stations uses the mapping pattern shown in FIG. 8, theother base station may also use an mapping pattern shown in FIG. 10. Inthe mapping pattern shown in FIG. 10, R4 to R7 are mapped to RBs 1, 2,3, 0 and 1 in order of sub-frames 0, 1, 2, 3 and 4. That is, in themapping pattern shown in FIG. 8, R4 to R7 are mapped to RBs shifted byone RB from RB0 in sub-frame 0, while in the mapping pattern shown inFIG. 10, R4 to R7 are mapped to RBs shifted by one RB from RB1 insub-frame 0. This can reduce the probability that R4 to R7 may be mappedto the same time domain and the same frequency domain in a plurality ofcells. Cell-specific RSs are generally transmitted to all terminals in acell, and are therefore transmitted with greater transmission power thanthat of data symbols. That is, a terminal located on the cell boundaryreceives not only cell-specific RSs from the cell to which the terminalbelongs but also cell-specific RSs from neighboring cells, and thereforeinterference between cell-specific RSs of different cells increases.However, as described above, using mapping patterns whose time domainand frequency domain differ from one cell to another makes it possibleto reduce interference between cell-specific RSs of different cells andthereby improves the accuracy of CQI measurement and PMI estimation ineach terminal.

Furthermore, the present invention may also be adapted such that oneframe is comprised of four sub-frames and one frame constitutes onecycle of an mapping pattern in which R4 to R7 are mapped to all RBs. Inthis case, an LTE+ terminal that has moved from a neighboring cell dueto handover or the like can receive cell-specific RSs (R4 to R7) withoutknowing frame numbers.

<Mapping Method 2 (FIG. 11)>

While mapping method 1 maps cell-specific RSs used only for LTE+terminals to one RB in the same sub-frame, the present mapping methodmaps cell-specific RSs used only for LTE+ terminals to a plurality ofRBs in the same sub-frame.

When the terminal moves slow, the fluctuation of channel quality betweenthe base station and the terminal becomes slow. On the other hand, whenthe terminal moves faster, the fluctuation of channel quality betweenthe base station and the terminal becomes more intense. That is, whenthe terminal moves faster, the fluctuation of channel quality persub-frame becomes more intense. Thus, when the terminal moves faster,use of an RS acquired in a sub-frame preceding by a long time intervalprevents the channel quality at the current point in time from beingcorrectly reflected, causing the accuracy of CQI measurement and PMIestimation to deteriorate.

Thus, according to the present mapping method, cell-specific RSs (R4 toR7) used only for LTE+ terminals in the same sub-frame are mapped to aplurality of RBs.

To be more specific, as shown in FIG. 11, R4 to R7 in sub-frame 0 aremapped to RB0 and RB1, R4 to R7 in sub-frame 1 are mapped to RB2 andRB3, R4 to R7 in sub-frame 2 are mapped to RB0 and RB1, R4 to R7 insub-frame 3 are mapped to RB2 and RB3 and R4 to R7 in sub-frame 4 aremapped to RB0 and RB1.

That is, as shown in FIG. 11, setting section 105 (FIG. 4) of basestation 100 sets two RBs, RB0 and RB1, in sub-frame 0 and two RBs, RB2and RB3, in sub-frame 1 as RBs to which cell-specific RSs used only forLTE+ terminals (R4 to R7) are mapped. The same applies to sub-frames 2to 4 as well.

Furthermore, as shown in FIG. 7, mapping section 106 maps R4 to R7 tocorresponding REs in RB0 and corresponding REs in RB1 in sub-frame 0respectively and maps R4 to R7 to corresponding REs in RB2 andcorresponding REs in RB3 in sub-frame 1 respectively. The same appliesto sub-frames 2 to 4 as well.

As shown in FIG. 11, R4 to R7 are mapped to ten RBs out of twenty RBs inone frame. That is, only R0 to R3 that can be received by LTE terminalsare transmitted on ten RBs (“4 RSs” shown in FIG. 11) other than someRBs (“8 RSs” shown in FIG. 11) to which R4 to R7 are mapped. Thus, LTEterminals can prevent deterioration of error rate characteristics in thesame way as with mapping method 1 (FIG. 8).

Furthermore, according to mapping method 1 (FIG. 8), terminal 200 (LTE+terminal) can receive cell-specific RSs (R0 to R7) of all RBs in foursub-frames, while in FIG. 11, terminal 200 (LTE+ terminal) can receivecell-specific RSs (R0 to R7) of all RBs in two sub-frames. In otherwords, according to mapping method 1 (FIG. 8), terminal 200 (LTE+terminal) can receive R4 to R7 every four sub-frames in the same RB,while in FIG. 11, terminal 200 (LTE+ terminal) can receive R4 to R7every two sub-frames in the same RB. That is, terminal 200 (LTE+terminal) can receive new R4 to R7 at shorter sub-frame intervals thanmapping method 1. Thus, the present mapping method can update channelquality for all RBs at shorter sub-frame intervals than mappingmethod 1. In this way, even when terminal 200 (LTE+ terminal) movesfast, it is possible to use channel quality measured using cell-specificRSs in a sub-frame, the reception time of which is newer, and thereforeterminal 200 can improve the accuracy of CQI measurement and PMIestimation.

The present mapping method may also use an mapping pattern shown in FIG.12 instead of the mapping pattern shown in FIG. 11. That is,cell-specific RSs used only for LTE+ terminals (R4 to R7) may be mappedto a plurality of discontinuous RBs in the frequency domain in the samesub-frame.

To be more specific, as shown in FIG. 12, in sub-frame 0, R4 to R7 aremapped to RB0 and to RB2 which is discontinuous to RB0 in the frequencydomain, while in sub-frame 1, R4 to R7 are mapped to RB1 and to RB3which is discontinuous to RB1 in the frequency domain. The same appliesto sub-frames 2 to 4 as well.

By mapping cell-specific RSs used only for LTE+ terminals to a pluralityof discontinuous RBs in the frequency domain in the same sub-frame, RBs(“4 RSs” shown in FIG. 12) to which base station 100 can allocate datasignals directed to LTE terminals also become discontinuous in thefrequency domain. Thus, even when frequency selectivity is slow, basestation 100 can allocate RBs which are distributed in the frequencydomain to LTE terminals. This prevents base station 100 fromcontinuously allocating LTE terminals to RBs of low receiving quality,and can thereby improve the frequency scheduling effect.

In the present mapping method, the number of RBs to which LTE terminalscan be allocated decreases compared to mapping method 1 (FIG. 8).However, since RBs to which LTE terminals can be allocated vary from onesub-frame to another, base station 100 can allocate LTE terminals to RBswith high channel quality in one of two continuous sub-frames. That is,the deterioration in the frequency scheduling effect due to thereduction in the number of RBs to which LTE terminals can be allocatedis small.

Thus, according to the present mapping method, cell-specific RSs usedonly for LTE+ terminals are mapped to part of a plurality of RBs in thesame sub-frame. This provides similar effects to those of mapping method1 to be obtained. Furthermore, according to the present mapping method,even when LTE+ terminals that move fast are present, the LTE+ terminalscan perform CQI measurement and PMI estimation using RSs received innewer sub-frames—that is, RSs in which channel quality at the currentpoint in time is reflected.

According to the present mapping method, base station 100 may switchbetween the mapping pattern shown in FIG. 11 and the mapping patternshown in FIG. 12 in accordance with the situation (frequencyselectivity) of the propagation path in a cell. That is, setting section105 of base station 100 may change the frequency interval of a pluralityof RBs in the same sub-frame to which R4 to R7 are mapped in accordancewith the condition of the propagation path in the cell. This allows basestation 100 to perform scheduling that matches the situation of thepropagation path, and can thereby further improve the frequencyscheduling effect.

<Mapping Method 3 (FIG. 13)>

According to the present mapping method, cell-specific RSs used only forLTE+ terminals are mapped to part of RBs at predetermined sub-frameintervals.

As described above, when a terminal moves slow, the fluctuation ofchannel quality between the base station and the terminal becomesslower. Thus, when the terminal moves slow, the accuracy of CQImeasurement and PMI estimation does not deteriorate even when thechannel quality obtained using RSs acquired in a sub-frame preceding bya long time interval is used as channel quality at the current point intime. Thus, when the terminal moves slow, cell-specific RSs used onlyfor LTE+ terminals need not be mapped to RBs every sub-frame as in thecase of mapping method 1 (FIG. 8).

Thus, the present mapping method maps cell-specific RSs used only forLTE+ terminals (R4 to R7) to part of RBs at predetermined sub-frameintervals.

In the following descriptions, it is assumed that the predeterminedsub-frame interval is two sub-frames. Furthermore, cell-specific RSsused only for LTE+ terminals (R4 to R7) are mapped to a plurality ofdiscontinuous RBs in the frequency domain in the same sub-frame in thesame way as with mapping method 2 (FIG. 12).

To be more specific, as shown in FIG. 13, R4 to R7 are mapped to RB0 andRB2 in sub-frame 0, R4 to R7 are mapped to RB1 and RB3 in sub-frame 2 atan interval of two sub-frames from sub-frame 0 and R4 to R7 are mappedto RB0 and RB2 in sub-frame 4 at an interval of two sub-frames fromsub-frame 2.

That is, as shown in FIG. 13, setting section 105 (FIG. 4) of basestation 100 sets two RBs, RB0 and RB2, in sub-frame 0, sets two RBs, RB1and RB3, in sub-frame 2 and sets two RBs, RB0 and RB2, in sub-frame 4 asRBs to which cell-specific RSs used only for LTE+ terminals (R4 to R7)are mapped. On the other hand, setting section 105 does not set any RB,to which R4 to R7 are mapped, in sub-frame 1 and sub-frame 3.

Furthermore, as shown in FIG. 7, mapping section 106 maps R4 to R7 tocorresponding REs in RB0 and to corresponding REs in RB2 in sub-frame 0respectively, maps R4 to R7 to corresponding REs in RB1 and tocorresponding REs in RB3 in sub-frame 2 respectively and maps R4 to R7to corresponding REs in RB0 and to corresponding REs in RB2 in sub-frame4 respectively.

As shown in FIG. 13, R4 to R7 are mapped to only six RBs out of twentyRBs in one frame. That is, only R0 to R3 that can be received by LTEterminals are transmitted on fourteen RBs (“4 RSs” shown in FIG. 13)other than some RBs (“8 RSs” shown in FIG. 13) to which R4 to R7 aremapped. Thus, LTE terminals can prevent deterioration of error ratecharacteristics in the same way as with mapping method 1 (FIG. 8).

Furthermore, in FIG. 13, terminal 200 (LTE+ terminal) can receivecell-specific RSs (R0 to R7) of all RBs in four sub-frames. Thus,terminal 200 (LTE+ terminal) can update CQI and PMI for each RB everyfour sub-frames in the same way as with mapping method 1 (FIG. 8).

In this way, according to the present mapping method, cell-specific RSsused only for LTE+ terminals are mapped to part of RBs at predeterminedsub-frame intervals. It is thereby possible to reduce the number ofcell-specific RSs used only for LTE+ terminals in one frame whilemaintaining the accuracy of CQI measurement and PMI estimation in LTE+terminals and increase the number of RBs to which data signals directedto LTE terminals are allocated. Thus, according to the present mappingmethod, even when LTE terminals and LTE+ terminals coexist, it ispossible to secure as many RBs as possible to be allocated to LTEterminals and thereby prevent deterioration in the throughput of LTEterminals in the same way as with mapping method 1.

The present mapping method assumes the predetermined sub-frame intervalto be two sub-frames, but the predetermined sub-frame interval is notlimited to two sub-frames. For example, base station 100 may set thepredetermined sub-frame interval according to the speed at which LTE+terminals move. To be more specific, the slower the LTE+ terminals move,the slower is the fluctuation of channel quality, and therefore basestation 100 may increase the predetermined sub-frame interval.Furthermore, the predetermined sub-frame interval may be notifiedthrough RRC signaling per terminal or broadcast per cell.

Mapping methods 1 to 3 of the present embodiment have been described sofar.

Thus, even when LTE terminals and LTE+ terminals coexist, the presentembodiment can prevent deterioration in the throughput of LTE terminals.Furthermore, according to the present embodiment, the base station canperform frequency scheduling for more frequency bands because there areno more scheduling constraints on RBs to which LTE+ terminals areallocated and the number of RBs to which LTE terminals are allocatedincreases.

The present embodiment has described a case where the number ofsub-frames, of which one frame is comprised, is five and a plurality ofsubcarriers in one sub-frame are divided into four RBs. However, withthe present invention, the number of sub-frames, of which one frame iscomprised, is not limited to five and the number of RBs into which theplurality of subcarriers are divided in one sub-frame is not limited tofour.

Embodiment 2

The present embodiment will describe a case where mapping methods 1 to 3of Embodiment 1 are switched in accordance with a cell environment.

As described above, mapping method 1 can reduce the number of RBs towhich cell-specific RSs used only for LTE+ terminals (R4 to R7) aremapped compared to mapping method 2. On the other hand, mapping method 2allows the base station to transmit cell-specific RSs (R4 to R7) in allRBs at shorter sub-frame intervals than that of mapping method 1. Thatis, mapping method 1 can secure more RBs to which LTE terminals areallocated in one frame than mapping method 2, whereas mapping method 2can shorten the sub-frame interval at which LTE+ terminals can updatechannel quality for all frequency domains compared to mapping method 1.

Similarly, mapping method 3 can secure more RBs to which LTE terminalsare allocated in one frame than mapping method 2, whereas mapping method2 can shorten the sub-frame interval at which LTE+ terminals can updatechannel quality for all frequency domains compared to mapping method 3.

That is, mapping method 1 (mapping method 3) and mapping method 2 have atrade-off relationship between the number of RBs in which LTE terminalscan be allocated in one frame and a sub-frame interval at which LTE+terminals can update channel quality for all RBs.

Thus, setting section 105 (FIG. 4) according to the present embodimentswitches between mapping method 1 (mapping method 3) and mapping method2 of Embodiment 1 in accordance with a cell environment and sets RBs towhich cell-specific RSs (R4 to R7) are mapped.

Hereinafter, switching methods 1 and 2 by setting section 105 of thepresent embodiment will be described.

<Switching Method 1>

The present switching method switches the mapping method according tothe number of LTE terminals in a cell.

As described above, base station 100 (FIG. 4) maps R4 to R7 which areterminal-specific RSs, and can thereby allocate LTE+ terminals to RBsother than RBs to which cell-specific RSs (R4 to R7) are mapped. Bycontrast, base station 100 can only allocate LTE terminals to RBs otherthan RBs to which cell-specific RSs (R4 to R7) are mapped. Therefore, asthe number of LTE terminals increases, base station 100 has to securemore RBs to which LTE terminals can be allocated—that is, RBs other thanRBs to which cell-specific RSs used only for LTE+ terminals are mapped.In other words, as the number of LTE terminals increases, base station100 has to reduce the number of RBs to which cell-specific RSs used onlyfor LTE+ terminals are mapped.

On the other hand, as the number of LTE terminals decreases, basestation 100 can secure more RBs to which cell-specific RSs used only forLTE+ terminals are mapped. This allows terminal 200 (FIG. 5) to receivecell-specific RSs used only for LTE+ terminals at more RBs and therebyimproves the frequency scheduling effect of LTE+ terminals.

Thus, when there are more LTE terminals, setting section 105 sets RBs towhich R4 to R7 are mapped using mapping method 1 (mapping method 3) andsets, when there are fewer LTE terminals, RBs to which R4 to R7 aremapped using mapping method 2. To be more specific, setting section 105compares the number of LTE terminals and a preset threshold, andswitches the mapping method. That is, setting section 105 switches tomapping method 1 (mapping method 3) when the number of LTE terminals isequal to or greater than the threshold and switches to mapping method 2when the number of LTE terminals is less than the threshold. That is,setting section 105 changes the number of cell-specific RSs used onlyfor LTE+ terminals according to the number of LTE terminals in a cell.

Thus, when the number of LTE terminals is large, base station 100 usesmapping method 1 (mapping method 3) and can thereby secure as many RBsas possible to which LTE terminals can be allocated while mappingcell-specific RSs used only for LTE+ terminals to part of RBs. On theother hand, when the number of LTE terminals is small, base station 100uses mapping method 2 and can thereby secure as many RBs as possible towhich cell-specific RSs used only for LTE+ terminals are mapped whilesecuring RBs to which LTE terminals can be allocated.

By this means, according to the present switching method, when thenumber of LTE terminals in a cell is large, the base station switches toan mapping method that allows RBs to which LTE terminals can beallocated to be obtained preferentially. On the other hand, when thenumber of LTE terminals in the cell is small, the base station shortensthe sub-frame interval at which LTE+ terminals can receive cell-specificRSs in all frequency bands and thereby switches to an mapping methodwhereby the frequency scheduling effect can be obtained preferentially.In this way, regardless of whether the number of LTE terminals in thecell is large or small, it is possible to obtain the frequencyscheduling effect in LTE+ terminals while securing RBs to which LTEterminals are allocated.

<Switching Method 2>

The present switching method switches an mapping method according to themoving speed of LTE+ terminals in a cell.

As described above, the higher the moving speed of an LTE+ terminal, themore intense is the fluctuation of channel quality, and thereforeterminal 200 has to update channel quality for each RB at shorter timeintervals—that is, at shorter sub-frame intervals—to perform CQImeasurement and PMI estimation without deteriorating the accuracy.

On the other hand, the lower the moving speed of an LTE+ terminal, theslower is the fluctuation of channel quality, and therefore terminal 200can perform CQI measurement and PMI estimation without deteriorating theaccuracy even when channel quality for each RB is updated at a longertime interval—that is, at longer sub-frame intervals.

Thus, when the moving speed of the LTE+ terminal is low, setting section105 sets RBs to which R4 to R7 are mapped using mapping method 1(mapping method 3) and sets RBs to which R4 to R7 are mapped usingmapping method 2 when the moving speed of the LTE+ terminal is high. Tobe more specific, setting section 105 compares the moving speed of theLTE+ terminal with a preset threshold and switches the mapping method.That is, setting section 105 switches to mapping method 1 (mappingmethod 3) when there are only LTE+ terminals whose moving speed is equalto or below the threshold, and switches to mapping method 2 when thereare only LTE+ terminals whose moving speed is greater than thethreshold. That is, setting section 105 changes the sub-frame intervalat which cell-specific RSs used only for LTE+ terminals are mappedaccording to the moving speed of LTE+ terminals.

In this way, when the moving speed of an LTE+ terminal is low, basestation 100 uses mapping method 1 (mapping method 3), and can therebyreduce the number of RBs to which cell-specific RSs used only for LTE+terminals are mapped to a necessary minimum and secure as many RBs aspossible to which LTE terminals can be allocated. On the other hand,when the moving speed of an LTE+ terminal is low, base station 100 usesmapping method 2, and can thereby secure the number of RBs to which LTEterminals can be allocated and secure as many RBs as possible to whichcell-specific RSs used only for LTE+ terminals are mapped.

By this means, according to the present switching method, when themoving speed of an LTE+ terminal in a cell is low, the base stationswitches to an mapping method whereby RBs to which LTE terminals can beallocated can be obtained preferentially. On the other hand, when themoving speed of the LTE+ terminal in the cell is high, the base stationshortens the sub-frame interval at which LTE+ terminals can receivecell-specific RSs in all frequency bands and thereby switches to anmapping method whereby a frequency scheduling effect can be obtainedpreferentially. Thus, whether the moving speed of the LTE+ terminal inthe cell is high or low, it is possible to obtain the frequencydiversity effect in the LTE+ terminal while securing the number of RBsto which LTE terminals are allocated in the same way as with switchingmethod 1.

Switching methods 1 and 2 by setting section 105 of the presentembodiment have been described so far.

Thus, the present embodiment switches between mapping methods forcell-specific RSs used only for LTE+ terminals in accordance with a cellenvironment. Thus, it is possible to obtain a maximum frequencyscheduling effect in LTE+ terminals while securing as many RBs aspossible to which LTE terminals can be allocated in accordance with thecell environment.

In the present embodiment, when switching between the mapping pattern ofmapping method 1 (mapping method 3) and the mapping pattern of mappingmethod 2, base station 100 (FIG. 4) may broadcast information indicatingthat the mapping pattern has been switched to all terminals 200 (LTE+terminals) using a BCH signal. Here, mapping patterns 1 to 3 are sharedbetween base station 100 and terminal 200. In this way, base station 100can switch between mapping patterns in accordance with a cellenvironment without the need of notifying the mapping pattern toterminal 200 every time the mapping pattern is switched. Furthermore,base station 100 may individually notify information indicating the factthat the mapping pattern has been switched to LTE+ terminals using RRC(Radio Resource Control) signaling.

Embodiment 3

3GPP-LTE defines, for example, the following three methods as methodsfor allocating LTE terminals to RBs. A first allocation method(hereinafter referred to as “type 0 allocation”) is a method whereby aplurality of RBs in a system band are grouped into a plurality of RBgroups and the base station allocates LTE terminals in units of RBgroups. Here, the number of RBs, of which an RB group is comprised,differs depending on the system bandwidth. Type 0 allocation has a highdegree of freedom of RB allocation, is suitable for transmission oflarge-volume data through frequency scheduling and allows highthroughput to be obtained.

A second allocation method (hereinafter referred to as “type 1allocation”) is a method whereby part of RB groups in the system bandare extracted and the base station allocates terminals in units of RBswithin extracted part of RB groups. According to type 1 allocation,although combinations of RBs simultaneously allocated to terminals arelimited, terminals are allocated in units of RBs and the granularity ofRB allocation becomes finer, and is therefore suitable for RB allocationfor terminals transmitting only a small amount of data.

A third allocation method (hereinafter referred to as “type 2allocation”) is a method whereby the base station allocates terminals tocontinuous RBs in the frequency domain. According to type 2 allocation,the base station has only to notify start points and end points of RBs,to which terminals are allocated, to the terminals, and the amount ofinformation for notifying the RB allocation result is therefore smaller.Furthermore, according to type 2 allocation, combinations of RBssimultaneously allocated to terminals are limited as in the case of type1 allocation, but since terminals are allocated in units of RBs, thegranularity of RB allocation becomes finer, and is therefore suitablefor RB allocation of terminals transmitting only a small amount of data.

Here, the base station cannot allocate LTE terminals to RBs to whichcell-specific RSs used only for LTE+ terminals (R4 to R7) are mapped.For this reason, according to type 0 allocation that performs RBallocation in units of RB groups, when RBs to which cell-specific RSsused only for LTE+ terminals are mapped are included in some of aplurality of RBs of which an RB group is comprised, the base stationcannot allocate LTE terminals to the RB group. That is, with type 0allocation, RB groups that can be allocated to LTE terminals are limitedand scheduling constraints on RBs to which LTE terminals are allocatedmay increase.

For example, FIG. 14 shows an example of RS mapping where cell-specificRSs used only for LTE+ terminals are mapped to RBs shifted by one RB inthe frequency domain every sub-frame. FIG. 14 describes an example wherea plurality of subcarriers are uniformly divided into eight RBs, RB0 toRB7, in one sub-frame. Furthermore, one RB group is comprised of two RBs(that is, RB group size: two RBs). For example, as shown in FIG. 14, RB0and RB1 constitute one RB group, RB2 and RB3 constitute one RB group,RB4 and RB5 constitute one RB group and RB6 and RB7 constitute one RBgroup. Furthermore, as in the case of Embodiment 1, as shown in FIG. 14,an RB to which four RSs, R0 to R3, are mapped (FIG. 6) is represented as“4 RSs” and an RB to which eight RSs, R0 to R7, are mapped (FIG. 7) isrepresented as “8 RSs.”

Here, in sub-frame 0 shown in FIG. 14, cell-specific RSs used only forLTE+ terminals (R4 to R7) are mapped to two RBs, RB0 and RB6. For thisreason, the base station cannot allocate LTE terminals to an RB groupincluding RB0 (RB group is comprised of RB0 and RB1 shown in FIG. 14)and an RB group including RB6 (RB group is comprised of RB6 and RB7shown in FIG. 14). Therefore, of eight RBs, RB0 to RB7, in sub-frame 0shown in FIG. 14, cell-specific RSs used only for LTE+ terminals aremapped to two RBs (RB0 and RB6), whereas LTE terminals cannot beallocated to four RBs (RB0, RB1, RB6 and RB7).

Thus, according to type 0 allocation, there may be RBs to which LTEterminals cannot be allocated though these are RBs to whichcell-specific RSs used for both LTE terminals and LTE+ terminals aremapped. Since type 0 allocation is an RB allocation method fit totransmit a large volume of data through frequency scheduling, schedulingconstraints on RBs to which LTE terminals are allocated have a greatinfluence on deterioration in the throughput of LTE terminals. Inparticular, when the RB group size is large, the deterioration in thethroughput of LTE terminals is greater.

Thus, the present embodiment maps cell-specific RSs used only for LTE+terminals to a plurality of RBs, of which part of RB groups iscomprised, in the same sub-frame in one frame.

Hereinafter, a cell-specific RS mapping method of the present embodimentwill be described.

In the following descriptions, as shown in FIG. 15, one RB group iscomprised of two RBs in the same way as FIG. 14 (RB group size: 2).

The present embodiment maps cell-specific RSs used only for LTE+terminals (R4 to R7) to a plurality of RBs, of which part of RB groupsis comprised, in the same sub-frame in one frame. To be more specific,as shown in FIG. 15, R4 to R7 are mapped to RB0 and RB1 constituting oneRB group in sub-frame 0, R4 to R7 are mapped to RB2 and RB3 constitutingone RB group in sub-frame 1, R4 to R7 are mapped to RB4 and RB5constituting one RB group in sub-frame 2 and R4 to R7 are mapped to RB6and RB7 constituting one RB group in sub-frame 3.

That is, as shown in FIG. 15, setting section 105 (FIG. 4) of basestation 100 sets the RB group comprised of RB0 and RB1 in sub-frame 0and sets the RB group comprised of RB2 and RB3 in sub-frame 1 as RBgroups to which cell-specific RSs used only for LTE+ terminals (R4 toR7) are mapped. The same applies to sub-frame 2 and sub-frame 3 as well.That is, setting section 105 sets RBs to which cell-specific RSs usedonly for LTE+ terminals are mapped in units of RB groups.

Mapping section 106 (FIG. 4) of base station 100 maps R4 to R7 to aplurality of RBs constituting the RB group set by setting section 105.That is, as shown in FIG. 7, mapping section 106 maps R4 to R7 tocorresponding REs in RB0 (and RB1) of sub-frame 0 respectively and mapsR4 to R7 to corresponding REs in RB2 (and RB3) of sub-frame 1respectively. The same applies to sub-frame 2 and sub-frame 3 as well.

As shown in FIG. 15, in each sub-frame, there is one RB group (that is,RB group including RBs to which cell-specific RSs used only for LTE+terminals are mapped) to which LTE terminals cannot be allocated throughtype 0 allocation. That is, in each sub-frame, the number of RBs towhich cell-specific RSs used only for LTE+ terminals are mapped is two,while the number of RBs to which LTE terminals cannot be allocatedthrough type 0 allocation is also two.

Thus, cell-specific RSs used only for LTE+ terminals are mapped in unitsof RB groups, and therefore the number of RBs to which cell-specific RSsused only for LTE+ terminals are mapped is equal to the number of RBs towhich LTE terminals cannot be allocated through type 0 allocation. Thatis, base station 100 can minimize the number of RBs to which LTEterminals cannot be allocated. It is thereby possible to minimizescheduling constraints on RBs to which LTE terminals are allocated andprevent deterioration in the throughput of LTE terminals.

By this means, even when terminals are allocated in units of RB groups,the present embodiment can prevent deterioration in the throughput ofLTE terminals in the same way as Embodiment 1. Type 0 allocation thatperforms RB allocation in units of RB groups in particular is an RBallocation method that can improve the throughput of LTE terminals thatperform high-speed transmission most. Thus, the present embodimentreduces scheduling constraints on RBs to which LTE terminals areallocated, and can thereby prevent any reduction in the number of RBs towhich LTE terminals are allocated in a cell in which LTE terminalsperforming high-speed transmission are accommodated.

The present embodiment has described a case where the base stationallocates terminals in units of RB groups. However, the base station ofthe present invention can provide similar effects to those of thepresent embodiment even when allocating terminals in units of integralmultiples of the RB group.

Furthermore, the present embodiment has described a case where thenumber of sub-frames, of which one frame is comprised, is four and aplurality of subcarriers are divided into eight RBs in one sub-frame.However, according to the present invention, the number of sub-frames,of which one frame is comprised, is not limited to four and the numberof RBs into which a plurality of subcarriers are divided in onesub-frame is not limited to eight.

Embodiment 4

Another RB allocation method other than the RB allocation (type 0allocation, type 1 allocation and type 2 allocation) described inEmbodiment 3 is distributed RB allocation (frequency hopping). Accordingto distributed RB allocation, the base station allocates one terminaldistributedly into a plurality of different RBs. A case will bedescribed below where one terminal is allocated distributedly into twodifferent RBs. That is, for example, each RB in one sub-frame istemporally divided into a first half part and a second half part in thetime domain and the base station allocates one terminal distributedlyinto the first half part of one of the two different RBs and the secondhalf part of the other RB. Furthermore, in distributed RB allocation, afrequency interval (RB interval, hopping interval or gap) between twodifferent RBs is predetermined based on the system bandwidth.Distributed RB allocation that can obtain a frequency diversity effectcompared to frequency scheduling whereby RBs with good quality areallocated every sub-frame is suitable for, for example, semi-persistentscheduling (SPS) intended for speech packet transmission which continuesto use RBs once allocated.

Here, according to distributed RB allocation, when the base stationallocates one LTE terminal, both two RBs to which the LTE terminal isallocated need to be RBs that can be allocated to the LTE terminal—thatis, RBs to which cell-specific RSs used for both LTE terminals and LTE+terminals are mapped. That is, according to distributed RB allocation,when RB to which cell-specific RSs used only for LTE+ terminals aremapped are included in the two RBs located by an RB intervalpredetermined through distributed RB allocation, the base station cannotallocate LTE terminals to the two RBs. That is, with distributed RBallocation, there may be cases where RBs that can be allocated to LTEterminals are limited, resulting in an increase of schedulingconstraints on RBs to which LTE terminals are allocated.

FIG. 16 shows an example of RS mapping where cell-specific RSs used onlyfor LTE+ terminals are mapped to RBs shifted by one RB in the frequencydomain every sub-frame. In FIG. 16, it is assumed that the systembandwidth is sixteen RBs (RB0 to RB15) and the RB interval (hoppinginterval) between two RBs in distributed RB allocation is eight RBs.Furthermore, as in the case of Embodiment 1, as shown in FIG. 16, RBs(FIG. 6) to which four RSs, R0 to R3, are mapped are represented by “4RSs” and RBs (FIG. 7) to which eight RSs, R0 to R7, are mapped arerepresented by “8 RSs.” Furthermore, in sub-frame 2 shown in FIG. 16,cell-specific RSs used only for LTE+ terminals (R4 to R7) are mapped tothree RBs, RB2, RB8 and RB14.

Here, a case is assumed where distributed RB allocation on a terminal isperformed in sub-frame 2 shown in FIG. 16 using RB10. In this case, thefirst half part or the second half part of RB10 and the second half partor the first half part of RB2 located by hopping interval eight RBs awayfrom RB10 are allocated to the terminal. However, since RB2 is an RB towhich cell-specific RSs used only for LTE+ terminals are mapped, thebase station cannot allocate LTE terminals to RB2 and RB10. Similarly,in addition to RB8 and RB14 to which cell-specific RSs used only forLTE+ terminals are mapped, the base station cannot allocate LTEterminals to RB0 located by eight RBs away from RB8 and RB6 by eight RBsaway from RB14 either. Thus, of the sixteen RBs, RB0 to RB15, insub-frame 2 shown in FIG. 16, cell-specific RSs used only for LTE+terminals are mapped to three RBs (RB2, RB8 and RB14), whereas LTEterminals cannot be allocated to six RBs (RBs enclosed by broken linesshown in FIG. 16).

Thus, in distributed RB allocation, there may be RBs to which LTEterminals cannot be allocated although these are RBs to whichcell-specific RSs used for both LTE terminals and LTE+ terminals aremapped. That is, according to distributed RB allocation, schedulingconstraints on RBs to which LTE terminals are allocated may increase,resulting in a decrease of the throughput of LTE terminals or the numberof speech terminals accommodated.

Thus, the present embodiment maps cell-specific RSs used only for LTE+terminals to a plurality of RBs located by an RB interval (hoppinginterval) away from each other in distributed RB allocation.

Hereinafter, the cell-specific RS mapping method of the presentembodiment will be described.

As shown in FIG. 17, a case will be described below as an example wherethe system bandwidth is assumed to be sixteen RBs (RB0 to RB15) as inthe case of in FIG. 16. Furthermore, suppose the RB interval indistributed RB allocation is eight RBs.

In the present embodiment, cell-specific RSs used only for LTE+terminals (R4 to R7) are mapped to two RBs located by an RB interval(eight RBs) away from each other in distributed RB allocation. To bemore specific, as shown in FIG. 17, in sub-frame 0, R4 to R7 are mappedto RB0 and RB8 located by eight RBs away from RB0. Similarly, as shownin FIG. 17, R4 to R7 are mapped to RB1 and RB9 located by eight RBs awayfrom RB1 in sub-frame 1, R4 to R7 are mapped to RB2 and RB10 located byeight RBs away from RB2 in sub-frame 2, and R4 to R7 are mapped to RB3and RB11 located by eight RBs away from RB3.

That is, as shown in FIG. 17, setting section 105 (FIG. 4) of basestation 100 sets RB0 and RB8 in sub-frame 0 and sets RB1 and RB9 insub-frame 1 as RBs to which cell-specific RSs used only for LTE+terminals (R4 to R7) are mapped. The same applies to sub-frame 2 andsub-frame 3 as well. That is, in each sub-frame, setting section 105sets two RBs to which cell-specific RSs used only for LTE+ terminals aremapped in RBs located by the same number of RBs as the RB interval(hopping interval) away from each other in distributed RB allocation.

As shown in FIG. 7, mapping section 106 (FIG. 4) of base station 100maps R4 to R7 to corresponding REs in RB0 (and RB8) of sub-frame 0 andmaps R4 to R7 to corresponding REs in RB1 (and RB9) of sub-frame 1. Thesame applies to sub-frame 2 and sub-frame 3 as well.

As shown in FIG. 17, the number of RBs to which LTE terminals cannot beallocated in each sub-frame through distributed RB allocation (that is,RBs to which cell-specific RSs used only for LTE+ terminals are mappedor RBs whose frequency interval from RB to which cell-specific RSs usedonly for LTE+ terminals are mapped is eight RBs) is two. That is, whilethe number of RBs to which cell-specific RSs used only for LTE+terminals are mapped in each sub-frame is two, the number of RBs towhich LTE terminals cannot be allocated through distributed RBallocation is also two.

Thus, cell-specific RSs used only for LTE+ terminals are mapped to aplurality of RBs located by an RB interval away from each other indistributed RB allocation. This causes the number of RBs to whichcell-specific RSs used only for LTE+ terminals are mapped to becomeequal to the number of RBs to which LTE terminals cannot be allocatedthrough distributed RB allocation. That is, base station 100 canminimize the number of RBs to which LTE terminals cannot be allocated.This can minimize scheduling constraints on RBs to which LTE terminalsare allocated and prevent deterioration in the throughput of LTEterminals.

By this means, even when allocating terminals to RBs through distributedRB allocation, the present embodiment can prevent deterioration in thethroughput of LTE terminals as in the case of Embodiment 1. Inparticular, distributed RB allocation is mainly used for speech packettransmission. Thus, the present embodiment reduces schedulingconstraints on RBs to which LTE terminals are allocated, and can therebyprevent the number of LTE terminals allocated—that is, the number ofspeech terminals to be accommodated—from reducing in a cell in which LTEterminals performing speech conversation are accommodated.

A case has been described in the present embodiment where cell-specificRSs used only for LTE+ terminals are mapped to a plurality of RBslocated by an RB interval away from each other in distributed RBallocation in each sub-frame. However, according to the presentinvention, cell-specific RSs used only for LTE+ terminals may also bemapped to a plurality of RBs located by 1/N (where, N is a positiveinteger) of an RB interval away from each other in distributed RBallocation in each sub-frame.

Furthermore, according to 3GPP-LTE, the RB interval (hopping interval)in distributed RB allocation is an integral multiple of the number ofRBs, of which an RB group is comprised. Thus, when base station 100performs distributed RB allocation on terminals in units of RB groups,cell-specific RSs used only for LTE+ terminals may be mapped in units ofRB groups in each sub-frame and may be mapped to RB groups located by anRB interval (hopping interval) away from each other in distributed RBallocation.

That is, base station 100 may map cell-specific RSs used only for LTE+terminals to resource blocks constituting a plurality of RB groupslocated by an RB interval (hopping interval) away from each other indistributed RB allocation in the same sub-frame in one frame.

Here, FIG. 18 shows an example of RS mapping when cell-specific RSs usedonly for LTE+ terminals are mapped to RBs shifted by one RB group in thefrequency domain every sub-frame in units of RB groups as in the case ofEmbodiment 3. One RB group is comprised of two RBs (that is, RB groupsize: two RBs). Furthermore, the RB interval (hopping interval) indistributed RB allocation is assumed to be eight RBs. That is, the RBinterval (eight RBs) in distributed RB allocation is four times (anintegral multiple of) the RB group size (two RBs).

Thus, as shown in FIG. 18, R4 to R7 are mapped to RB0 and RB1constituting one RB group in sub-frame 0 and in RB8 and RB9 constitutingan RB group located by eight RBs away from the RB group (RB0 and RB1).Similarly, as shown in FIG. 18, R4 to R7 are mapped to RB2 and RB3constituting one RB group in sub-frame 1 and in RB10 and RB11constituting an RB group located by eight RBs away from the RB group(RB2 and RB3). The same applies to sub-frame 2 and sub-frame 3 as well.

In the same way as in Embodiment 3, allocating terminals in units of RBgroups makes it possible to minimize scheduling constraints on RBs towhich LTE terminals are allocated. Furthermore, in the same way as thepresent embodiment, allocating terminals to RBs through distributed RBallocation makes it possible to minimize scheduling constraints on RBsto which LTE terminals are allocated. That is, the combination betweenEmbodiment 3 and the present embodiment of the present invention issuitable for use in an LTE-advanced system.

Furthermore, by using the RS mapping pattern shown in FIG. 18, it ispossible to transmit cell-specific RSs using a format common to a cellaccommodating LTE terminals that perform high-speed transmission usingRB allocation (type 0 allocation) in units of RB groups and a cellaccommodating LTE terminals that perform speech conversation usingdistributed RB allocation. This makes it possible to realizesimplification of the system.

A case has been described in the present embodiment where the number ofsub-frames, of which one frame is comprised, is four and a plurality ofsubcarriers are divided into sixteen RBs in one sub-frame. However, withthe present invention, the number of sub-frames, of which one frame iscomprised, is not limited to four and the number of RBs into which aplurality of subcarriers are divided in one sub-frame is not limited tosixteen.

Embodiment 5

In mobile communication, HARQ (Hybrid Automatic Repeat reQuest) isapplied to a packet (downlink data) transmitted from a base station to aterminal in a downlink and a response signal indicating an errordetection result of the packet (downlink data) is fed back to the basestation in an uplink. The terminal feeds back an ACK (Acknowledgment)signal when the packet (downlink data) includes no error or an NACK(Negative Acknowledgment) signal when there is some error to the basestation as a response signal. When a NACK signal is fed back from theterminal, the base station then retransmits a packet (downlinkdata)—that is, performs HARQ retransmission.

Furthermore, when a NACK signal is fed back from the terminal, there issynchronous/non-adaptive retransmission in which the base stationretransmits a packet (retransmission packet) using the same RBs as thoseat initial transmission, a predetermined time after receiving the NACKsignal. The synchronous/non-adaptive retransmission does not requiresignaling for notifying retransmission of a packet, and can therebysuppress overhead of a control signal on the retransmission packet.

However, when LTE terminals and LTE+ terminals coexist, although RBs towhich cell-specific RSs used for both LTE terminals and LTE+ terminalsare mapped are allocated for packets initially transmitted to LTEterminals, RBs to which cell-specific RSs used only for LTE+ terminalsare mapped may be allocated upon retransmission, a predetermined timeafter the initial transmission. In this case, the base station cannotretransmit any retransmission packet to LTE terminals.

Therefore, the present embodiment maps cell-specific RSs used only forLTE+ terminals to each RB (or RB group) at the same time interval (thatis, sub-frame interval) as the retransmission interval in HARQ (that is,a predetermined time).

Hereinafter, a cell-specific RS mapping method according to the presentembodiment will be described. In the following descriptions, thecell-specific RS mapping pattern set by setting section 105 of basestation 100 is different from that of Embodiment 1. Furthermore, supposethe retransmission interval in HARQ (synchronous/non-adaptiveretransmission) is four sub-frames. Furthermore, as shown in FIG. 19,cell-specific RSs used only for LTE+ terminals (R4 to R7) are mapped tofour RBs (or two RB groups) in the same sub-frame.

Thus, as shown in FIG. 19, R4 to R7 are mapped to RB0, RB1, RB8 and RB9in sub-frame 0 and sub-frame 4, after four sub-frames (retransmissioninterval in HARQ) from sub-frame 0. Similarly, as shown in FIG. 19, R4to R7 are mapped to RB2, RB3, RB10 and RB11 in sub-frame 1 and sub-frame5, four sub-frames (retransmission interval in HARQ retransmission) fromsub-frame 1. The same applies to RB4 to RB7 and RB12 to RB15 as well.

As shown in FIG. 19, cell-specific RSs used only for LTE+ terminals (R4to R7) are mapped to respective RBs at the same time interval as theretransmission interval in HARQ (that is, retransmission cycle). Inother words, cell-specific RSs used only for LTE+ terminals are nevermapped to respective RBs in a sub-frame, after a retransmission intervalin HARQ from a sub-frame to which cell-specific RSs used for both LTEterminals and LTE+ terminals are mapped. That is, cell-specific RSs usedfor both LTE terminals and LTE+ terminals are reliably mapped torespective RBs in a sub-frame, after a retransmission interval in HARQfrom a sub-frame to which cell-specific RSs used for both LTE terminalsand LTE+ terminals are mapped.

Thus, for example, a case is assumed where base station 100 initiallytransmits packets to LTE terminals using RB2 and RB3 in sub-frame 0shown in FIG. 19. In this case, even when a NACK signal is fed back froman LTE terminal, base station 100 can reliably retransmit aretransmission packet to the LTE terminal in sub-frame 4, after foursub-frames (retransmission interval in HARQ) from the sub-frame uponinitial transmission.

By this means, the present embodiment maps cell-specific RSs used onlyfor LTE+ terminals in respective RBs at the same time interval as theretransmission interval in HARQ (retransmission cycle). This preventsthe cell-specific RS mapping from blocking synchronous/non-adaptiveretransmission of LTE terminals, and can thereby prevent deteriorationin the throughput of LTE terminals.

The retransmission interval in HARQ is the same as the number of HARQprocesses. That is, when the retransmission interval in HARQ is eightsub-frames, there are eight HARQ processes per terminal. Therefore, thepresent invention may assume the time interval at which cell-specificRSs used only for LTE+ terminals are mapped to respective RBs (that is,transmission cycle of cell-specific RSs used only for LTE+ terminals) tobe a time interval to match the number of HARQ processes.

Furthermore, the present embodiment has described only firstretransmission of a packet. However, even upon second and subsequentretransmissions, the present invention likewise prevents RBs used forretransmission at each retransmission timing from overlapping RBswhereby cell-specific RSs used only for LTE+ terminals are transmitted.

Furthermore, a case has been described in the present embodiment wherecell-specific RSs used only for LTE+ terminals are mapped to respectiveRBs at the same time interval as the retransmission interval in HARQ(transmission cycle). However, according to the present invention, thetime interval at which cell-specific RSs used only for LTE+ terminalsare mapped to respective RBs may be an integral multiple of theretransmission interval in HARQ (transmission cycle) and may be 1/N(where N is a positive integer) of the retransmission interval in HARQ(transmission cycle). When the time interval at which cell-specific RSsused only for LTE+ terminals are mapped to respective RBs is an integralmultiple of the retransmission interval in HARQ, RBs used forretransmission may overlap RBs whereby cell-specific RSs used only forLTE+ terminals are transmitted. However, it is possible to reduce theprobability that RBs used for retransmission may overlap RBs wherebycell-specific RSs used only for LTE+ terminals are transmitted.

Furthermore, a case has been described in the present embodiment wherethe number of sub-frames constituting one frame is eight and a pluralityof subcarriers of one sub-frame are divided into sixteen RBs. However,with the present invention, the number of sub-frames, of which one frameis comprised, is not limited to eight and the number of RBs into which aplurality of subcarriers are divided in one sub-frame is not limited tosixteen.

Embodiment 6

In 3GPP-LTE, when feeding back CQIs to the base station, LTE+ terminalsreport the CQIs in units of sub-bands, each of which bundles a pluralityof RBs, in a predetermined cycle (hereinafter, “CQI reporting cycle”).For example, when there are four sub-bands in the system band, an LTE+terminal reports four CQIs indicating channel quality for respectivesub-bands and an average CQI indicating average channel quality for theentire system band to the base station in a CQI reporting cycle.

Furthermore, each LTE+ terminal measures channel quality for each ofRBs, of which each sub-band is comprised, using cell-specific RSs usedonly for LTE+ terminals (R4 to R7) and generates a CQI for the sub-band.That is, in order for the LTE+ terminal to generate a CQI for eachsub-band, it is necessary to measure channel quality for all RBs, ofwhich each sub-band is comprised.

Thus, the present embodiment maps cell-specific RSs used only for LTE+terminals to each RB at the same time interval as the CQI reportingcycle. Furthermore, in each sub-frame, the present embodiment mapscell-specific RSs used only for LTE+ terminals in units of sub-bands.

Hereinafter, a cell-specific RS mapping method according to the presentembodiment will be described. In the following descriptions, acell-specific RS mapping pattern set by setting section 105 of basestation 100 is different from that of Embodiment 1. Furthermore, asshown in FIG. 20, suppose the system band is sixteen RBs (RB0 to RB15)and one sub-band is comprised of four RBs (that is, sub-band size: fourRBs). To be more specific, as shown in FIG. 20, sub-band 0 is comprisedof RB0 to RB3, sub-band 1 is comprised of RB4 to RB7, sub-band 2 iscomprised of RB8 to RB11 and sub-band 3 is comprised of RB12 to RB15.Furthermore, suppose the CQI reporting cycle is four sub-frames.

Thus, as shown in FIG. 20, in RB0 to RB3 constituting sub-band 0, R4 toR7 are mapped in sub-frame 0 and in sub-frame 4, after four sub-frames(CQI reporting cycle) from sub-frame 0. Similarly, as shown in FIG. 20,in RB4 to RB7 constituting sub-band 1, R4 to R7 are mapped in sub-frame1 and in sub-frame 5, after four sub-frames (CQI reporting cycle) fromsub-frame 1. The same applies to RB8 to RB11 constituting sub-band 2 andRB12 to RB15 constituting sub-band 3.

As shown in FIG. 20, in each sub-frame, R4 to R7 are transmitted frombase station 100 to LTE+ terminals in units of sub-bands. This allowsLTE+ terminals to measure channel quality for all RBs constituting onesub-band in one sub-frame. Furthermore, as shown in FIG. 20, in eachsub-band, R4 to R7 are transmitted from base station 100 to LTE+terminals at intervals of four sub-frames, which is the CQI reportingcycle. That is, the transmission cycle of cell-specific RSs used onlyfor LTE+ terminals in each RB is the same as the CQI reporting cycle.Thus, the LTE+ terminals can measure channel quality for all RBs in allsub-bands 0 to 3 over four sub-frames which correspond to the CQIreporting cycle.

That is, the LTE+ terminals can generate CQIs for four sub-bands 0 to 3and an average CQI for the entire system band (RB0 to RB15 shown in FIG.20) in one CQI reporting cycle (4-sub-frame intervals). This allows theLTE+ terminals to report all CQIs for the entire system band in one CQIreporting cycle, thus making it possible to minimize the delay in CQIreporting.

When all CQIs for the entire system band are reported in one CQIreporting cycle, the data size of CQI is greater than that when all CQIsof the entire system band are reported in a plurality of CQI reportingcycles. Here, the greater the encoded data size, the greater is thecoding gain. Thus, when the LTE+ terminals report all CQIs in the entiresystem band in one CQI reporting cycle, the coding gain increases, andcoding efficiency of the CQIs thereby improves.

By this means, the present embodiment maps cell-specific RSs used onlyfor LTE+ terminals to each RB at the same time interval as a CQIreporting cycle and maps the cell-specific RSs in units of sub-bands ineach sub-frame. This makes it possible to provide similar effects tothose of Embodiment 1 and minimize the delay in CQI reporting.

With the present invention, cell-specific RSs used only for LTE+terminals may be mapped in units of sub-bands in each sub-frame andmapped at the same time interval as the CQI reporting cycle to eachsub-band. For example, as shown in FIG. 21 instead of FIG. 20, betweenneighboring sub-bands in the frequency domain, cell-specific RSs usedonly for LTE+ terminals (R4 to R7) may be mapped to discontinuous RBsand sub-frames in the time domain and frequency domain.

Furthermore, in the present embodiment, when a plurality of CQIreporting cycles are defined and one of the CQI reporting cycles isselected for each terminal, the base station may transmit cell-specificRSs used only for LTE+ terminals in one CQI reporting cycle of theplurality of CQI reporting cycles, for example, in the same transmissioncycle (time interval) as the most typical CQI reporting cycle.

Furthermore, according to the present embodiment, the CQI reportingcycle has only to be a cycle in which all CQIs for the sub-bands to bereported are reported, and, for example, LTE+ terminals may report CQIsfor respective sub-bands continuously and sequentially in the timedomain within the CQI reporting cycle.

Furthermore, a case has been described in the present embodiment whereLTE+ terminals report all CQIs generated in their respective sub-bands.However, according to the present invention, LTE+ terminals may alsoreport only CQIs of higher sub-bands having better channel quality amongall CQIs generated in the respective sub-bands.

Furthermore, the present invention may also map cell-specific RSs usedonly for LTE+ terminals in each sub-frame in units of a least commonmultiple of the number of RBs constituting a sub-band (sub-band size)and the number of RBs constituting an RB group (RB group size). In thiscase, it is possible to provide similar effects to those of the presentembodiment and also provide similar effects to those of Embodiment 3.Here, according to 3GPP-LTE, the sub-band size is an integral multipleof the RB group size. Thus, according to 3GPP-LTE, as described above,if cell-specific RSs used only for LTE+ terminals are mapped in units ofsub-bands, cell-specific RSs used only for LTE+ terminals are alwaysmapped in units of a least common multiple of the sub-band size and theRB group size.

Furthermore, a case has been described in the present embodiment wherein respective RBs, cell-specific RSs used only for LTE+ terminals aremapped in one sub-frame in a CQI reporting cycle. However, according tothe present invention, at each RB, cell-specific RSs used only for LTE+terminals may also be mapped in a plurality of sub-frames in a CQIreporting cycle. That is, the transmission cycle of cell-specific RSsused only for LTE+ terminals may be faster than the CQI reporting cycle.In this case, in respective RBs, LTE+ terminals can improve the accuracyof CQIs by obtaining an average value of channel quality measured inplurality of sub-frames.

Furthermore, according to 3GPP-LTE, the number of bits of a controlsignal that can be transmitted in an uplink control channel (e.g. PUCCH(Physical Dedicated Control Channel)) is limited. For this reason,3GPP-LTE is studying a mode (periodic UE selected sub-band feedback) inwhich CQIs are reported to the base station one CQI at a time every Nsub-frames. Here, suppose the cycle per N sub-frames in which a CQI isreported is a CQI reporting cycle. In this CQI reporting mode, a CQI forthe sub-band with the best channel quality in one band (bandwidth part:hereinafter referred to as “partial band”) which is one of M portionsinto which the system band is divided is reported in the CQI reportingcycle. Furthermore, the partial band which is a CQI reporting target ineach CQI reporting cycle is shifted every N sub-frames. That is, thesub-frame in which a CQI is measured (CQI measurement sub-frame) differsfrom one partial band to another. In order to apply the presentinvention to this CQI reporting mode, cell-specific RSs used only forLTE+ terminals may be mapped in units of sub-bands included in eachpartial band in each sub-frame and mapped in a cycle M times the CQIreporting cycle (N sub-frames) per partial band ((N×M) sub-frames) ineach sub-band. That is, in each RB, the time interval at whichcell-specific RSs used only for LTE+ terminals are mapped may be assumedto be M times the CQI reporting cycle in a partial band. For example,FIG. 22 shows an example of RS mapping assuming N=4 and M=2—that is, acase where the CQI reporting cycle per partial band is four sub-framesand the entire system band is divided into partial band 0 and partialband 1. Furthermore, in FIG. 22, sub-frames 0 to 3 are assumed to be CQImeasurement sub-frames for partial band 0 and a CQI relating to partialband 0 is reported after a lapse of a predetermined time required forCQI measurement and preparation for transmission. Furthermore,sub-frames 4 to 7 are assumed to be CQI measurement sub-frames forpartial band 1 and a CQI relating to partial band 1 is reported after alapse of a similar predetermined time to that of partial band 0. Thatis, the CQI reporting cycle is four sub-frames. In this case, as shownin FIG. 22, in partial band 0 (sub-band 0 and sub-band 1), cell-specificRSs used only for LTE+ terminals (R4 to R7) are mapped in sub-frame 0and sub-frame 2 of sub-frames 0 to 3 which are CQI measurementsub-frames for partial band 0. Furthermore, in partial band 1 (sub-band2 and sub-band 3), cell-specific RSs used only for LTE+ terminals (R4 toR7) are mapped in sub-frame 4 and sub-frame 6 of sub-frames 4 to 7 whichare CQI measurement sub-frames for partial band 1. In sub-frame 8 andsubsequent sub-frames shown in FIG. 22, mapping of cell-specific RSs insub-frames 0 to 7 is repeated. That is, in FIG. 22, in each RB, the timeinterval at which cell-specific RSs used only for LTE+ terminals aremapped is eight sub-frames which is M (=two partial bands) times CQIreporting cycle N (=four sub-frames) per partial band.

Furthermore, a case has been described in the present embodiment wherethe number of sub-frames, of which one frame is comprised, is 8 and aplurality of subcarriers are divided into sixteen RBs in one sub-frame.However, according to the present invention, the number of sub-frames,of which one frame is comprised, is not limited to eight and the numberof RBs into which a plurality of subcarriers are divided in onesub-frame is not limited to sixteen.

Embodiment 7

In 3GPP-LTE, a base station allocates some LTE terminals to RBs throughSPS that continues to use once allocated RBs in a predetermined cycle(time interval). Here, the transmission cycle of RBs to which LTEterminals are allocated by the SPS is called “SPS transmission cycle.”By allocating LTE terminals to RBs through SPS, the base station doesnot have to notify control information indicating an RB allocationresult to LTE terminals every time transmission data is transmitted.

However, when LTE terminals and LTE+ terminals coexist, even whentransmission data directed to LTE terminals is allocated to RBs to whichcell-specific RSs used for both LTE terminals and LTE+ terminals aremapped at a certain transmission timing of the SPS transmission cycle,transmission data directed to LTE terminals may be allocated to RBs towhich cell-specific RSs used only for LTE+ terminals are mapped atanother transmission timing of the SPS transmission cycle. In this case,the base station cannot further transmit transmission data to LTEterminals allocated through SPS.

Thus, the present embodiment maps cell-specific RSs used only for LTE+terminals to each RB (or RB group) at a time interval 1/N (where, N is apositive integer) of the SPS transmission cycle.

Hereinafter, a cell-specific RS mapping method of the present embodimentwill be described. In the following descriptions, a cell-specific RSmapping pattern set by setting section 105 of base station 100 isdifferent from that of Embodiment 1. Furthermore, suppose the SPStransmission cycle is eight sub-frames. That is, transmission datadirected to a terminal allocated through SPS is transmitted every eightsub-frames. Furthermore, as shown in FIG. 23, cell-specific RSs usedonly for LTE+ terminals (R4 to R7) are mapped to four RBs (or two RBgroups) in the same sub-frame.

Thus, as shown in FIG. 23, R4 to R7 are mapped to RB0, RB1, RB8 and RB9in sub-frame 0 and sub-frame 8, after eight sub-frames (SPS transmissioncycle) from sub-frame 0. Similarly, as shown in FIG. 23, R4 to R7 aremapped to RB2, RB3, RB10 and RB11 in sub-frame 2 and sub-frame 10, aftereight sub-frames (SPS transmission cycle) from sub-frame 2. The sameapplies to RB4 to RB7 and RB12 to RBIS as well.

As shown in FIG. 23, cell-specific RSs used only for LTE+ terminals (R4to R7) are mapped to each RB at the same time interval as the SPStransmission cycle (eight sub-frames in FIG. 23). In other words, ineach RB, cell-specific RSs used only for LTE+ terminals are never mappedin a sub-frame, an SPS transmission cycle after the sub-frame to whichcell-specific RSs used for both LTE terminals and LTE+ terminals aremapped. That is, in each RB, cell-specific RSs used for both LTEterminals and LTE+ terminals are reliably mapped in a sub-frame, an SPStransmission cycle after the sub-frame to which cell-specific RSs usedfor both LTE terminals and LTE+ terminals are mapped.

Thus, for example, if base station 100 transmits transmission data toLTE terminals allocated through SPS using RB2 and RB3 in sub-frame 0shown in FIG. 23, it is possible to reliably transmit transmission datato LTE terminals from the next SPS transmission timing onward (e.g.sub-frame 8 shown in FIG. 23).

By this means, the present embodiment maps cell-specific RSs used onlyfor LTE+ terminals to each RB at the same time interval as the SPStransmission cycle. This prevents RBs allocated to LTE terminals throughSPS from being mixed with RBs to which cell-specific RSs used only forLTE+ terminals are mapped. It is thereby possible to preventcommunication quality of LTE terminals allocated through SPS fromdeteriorating and prevent deterioration in the throughput of LTEterminals.

A case has been described in the present embodiment where cell-specificRSs used only for LTE+ terminals are mapped to each RB at the same timeinterval (transmission cycle of eight RSs shown in FIG. 23) as the SPStransmission cycle. However, with the present invention, the timeinterval at which cell-specific RSs used only for LTE+ terminals aremapped to each RB may also be 1/N (e.g. 4-sub-frame intervals or2-sub-frame intervals in FIG. 23) of the SPS transmission cycle.

Furthermore, a case has been described in the present embodiment wherethe number of sub-frames, of which one frame is comprised, is eleven anda plurality of subcarriers are divided into sixteen RBs in onesub-frame. However, according to the present invention, the number ofsub-frames, of which one frame is comprised, is not limited to elevenand the number of RBs into which the plurality of subcarriers aredivided in one sub-frame is not limited to sixteen.

Embodiment 8

In 3GPP-LTE, broadcast information can be classified into threecategories based on the way physical resources are used; MIB (MasterInformation Block), SIB (System Information Block) 1 and SIB 2 to SIB 11(that is, SIBs from SIB 2 onward).

To be more specific, an MIB is transmitted in a fixed sub-frame (e.g.,sub-frame 0) and through P-BCH (Physical Broadcast Channel) using fixedfrequency resources. Furthermore, SIB 1 is transmitted in a fixedsub-frame (e.g., sub-frame 5 every two frames). Furthermore, SIBs fromSIB 2 onward are transmitted in one of transmittable sub-frames(SI-windows) indicated in scheduling information included in SIB 1. Inthe case of SIBs from SIB 2 onward, a sub-frame in which the SIBs aretransmitted is indicated in a downlink control channel (e.g. PDCCH(Physical Dedicated Control Channel)) notified in the sub-frame. Thatis, the terminal does not know in which sub-frame SIBs from SIB 2 onwardare transmitted until PDCCH is received in the sub-frame. PDCCH alsoincludes information indicating which RBs are used to transmit SIBs fromSIB 2 onward.

Here, since the above described broadcast information needs to bereceived by both LTE terminals and LTE+ terminals, when the broadcastinformation is transmitted using RBs to which cell-specific RSs usedonly for LTE+ terminals are mapped, the LTE terminals cannot furtherreceive the broadcast information.

Thus, the present embodiment maps cell-specific RSs used only for LTE+terminals according to sub-frames and RBs to which broadcast informationis allocated.

Hereinafter, a cell-specific RS mapping method according to the presentembodiment will be described. In the following descriptions, acell-specific RS mapping pattern set by setting section 105 of basestation 100 is different from that of Embodiment 1.

First, a sub-frame in which MIB or SIB 1 is transmitted will bedescribed.

Cell-specific RSs used only for LTE+ terminals are not mapped in thesub-frame in which MIB or SIB 1 is transmitted. That is, cell-specificRSs used only for LTE+ terminals are mapped in sub-frames other than thesub-frame in which MIB or SIB1 is transmitted (broadcast informationtransmission sub-frame). For example, as shown in FIG. 24, whenbroadcast information (MIB or SIB 1) is transmitted in sub-frame 1, R4to R7 are mapped in sub-frames other than sub-frame 1—that is,sub-frames 0, 2 to 7 in FIG. 24. That is, R4 to R7 are not mapped insub-frame 1 shown in FIG. 24. In FIG. 24, R4 to R7 are mapped in RBsshifted two RBs in the frequency domain in sub-frames 0, 2 to 7 otherthan the sub-frame in which broadcast information is transmitted.

Next, sub-frames in which SIBs from SIB 2 onward are transmitted will bedescribed.

In sub-frames in which SIBs from SIB 2 onward are transmitted,cell-specific RSs used only for LTE+ terminals are mapped to RBs in thesame way as, for example, Embodiment 3 (FIG. 15) or Embodiment 4 (FIG.18). On the other hand, SIBs from SIB 2 onward are transmitted using RBsother than RBs to which cell-specific RSs used only for LTE+ terminalsare mapped.

In this way, cell-specific RSs used only for LTE+ terminals are mappedin sub-frames other than the sub-frame in which MIB or SIB 1 istransmitted. Since the sub-frame in which MIB or SIB1 is transmitted isknown to LTE+ terminals, the LTE+ terminals may be adapted so as not toperform CQI measurement in the sub-frame in which MIB or SIB1 istransmitted.

Furthermore, since cell-specific RSs used only for LTE+ terminals arenot mapped in the sub-frame in which MIB or SIB 1 that needs to bereceived by both LTE terminals and LTE+ terminals is transmitted, it ispossible to secure more RBs that can be used to transmit broadcastinformation. Thus, base station 100 transmits broadcast information byencoding it at a sufficiently low coding rate in a sub-frame in whichbroadcast information is transmitted, and can thereby prevent error ratecharacteristics of the broadcast information from deteriorating.

By contrast, as for SIBs from SIB 2 onward, SIBs from SIB 2 onward aretransmitted using RBs other than RBs to which cell-specific RSs usedonly for LTE+ terminals are mapped. Here, sub-frames in which SIBs fromSIB 2 onward are transmitted are unknown to LTE+ terminals. However,according to the present embodiment, LTE+ terminals can perform normalCQI measurement regardless of whether or not the sub-frame is one inwhich SIBs from SIB 2 onward are transmitted. Therefore, LTE+ terminalsdo not have to decide whether or not to perform CQI measurement afterreceiving PDCCH, and can thereby simplify terminal processing and reducedelays. Furthermore, since SIBs from SIB 2 onward are transmitted withRBs to which cell-specific RSs used for both LTE terminals and LTE+terminals are mapped, LTE terminals can also reliably receive broadcastinformation.

Sub-frames in which broadcast information (broadcast information SIB+directed to LTE+ terminals) that needs to be received by only LTE+terminals, as opposed to the aforementioned broadcast information thatneeds to be received by both LTE terminals and LTE+ terminals, istransmitted are known to LTE+ terminals. Furthermore, the mapping ofcell-specific RSs used only for LTE+ terminals is also known to LTE+terminals. Thus, when broadcast information SIB+ directed to LTE+terminals is transmitted, it is not necessary to provide constraints onsub-frames (or RBs) to which cell-specific RS are mapped and sub-frames(or RBs) in which SIB+ is transmitted.

Thus, according to the present embodiment, both LTE terminals and LTE+terminals can reliably receive broadcast information and can alsoprevent error rate characteristics of broadcast information fromdeteriorating in sub-frames in which the broadcast information istransmitted.

A case has been described in the present embodiment where SIBs from SIB2 onward are transmitted using RBs other than RBs to which cell-specificRSs used only for LTE+ terminals are mapped. However, according to thepresent invention, SIBs from SIB 2 onward may also be transmitted, forexample, in sub-frames other than sub-frames to which cell-specific RSsused only for LTE+ terminals are mapped. Alternatively, cell-specificRSs used only for LTE+ terminals may also be mapped in sub-frames otherthan sub-frames in which SIBs from SIB 2 onward are transmitted based onan SI-window notified with SIB 1.

Furthermore, a case has been described in the present embodiment whereas shown in FIG. 24, cell-specific RSs used only for LTE+ terminals aremapped to RBs shifted every sub-frame not including a sub-frame in whichbroadcast information is transmitted (sub-frame 1 in FIG. 24). That is,a case has been described where cell-specific RSs used only for LTE+terminals are mapped to RBs shifted by two RBs in the frequency domainin sub-frames 0, 2 to 7 shown in FIG. 24. However, according to thepresent invention, as shown in FIG. 25, cell-specific RSs used only forLTE+ terminals may also be mapped to RBs shifted in the frequency domainevery sub-frame including a sub-frame in which broadcast information istransmitted (sub-frame 1 in FIG. 25). However, cell-specific RSs usedonly for LTE+ terminals are not mapped in a sub-frame in which broadcastinformation is transmitted. To be more specific, cell-specific RSs usedonly for LTE+ terminals are mapped to RBs shifted by two RBs in thefrequency domain in sub-frames 0 to 7 shown in FIG. 25. However,cell-specific RSs used only for LTE+ terminals are not mapped to RB2 andRB3 (RB10 and RB11) of sub-frame 1 in which broadcast information istransmitted. In this way, even when the sub-frame in which broadcastinformation is transmitted differs from one cell to another, RBs towhich cell-specific RSs used only for LTE+ terminals are mapped are thesame between the cells. Furthermore, RBs to which cell-specific RSs usedonly for LTE+ terminals are mapped become constant in a specific cycleregardless of the presence or absence of broadcast information. Thus, inthe same way as the present embodiment, LTE+ terminals located in eachcell do not measure CQI in a sub-frame in which broadcast information istransmitted, and the circuit necessary for CQI measurement of LTE+terminals can be simplified. Furthermore, to avoid interference betweenRSs between cells, when cell-specific RSs used only for LTE+ terminalsare mapped to different RBs between cells, the relationship between RBsto which cell-specific RSs used only for LTE+ terminals are mapped(mapping relationship of RBs to avoid interference) is maintainedbetween cells regardless of the presence or absence of broadcastinformation. This prevents the interference reduction effect fromdeteriorating.

Furthermore, according to the present embodiment, the mapping ofcell-specific RSs used only for LTE+ terminals may be avoided not onlyin MIB and SIB1 to SIB11 but also in sub-frames (MBSFN sub-frame) inwhich, for example, MBSFN (MBMS Single Frequency Network) data istransmitted. That is, cell-specific RSs used only for LTE+ terminals maybe mapped in sub-frames other than MBSFN sub-frames.

Furthermore, a case has been described in the present embodiment wherethe number of sub-frames, of which one frame is comprised, is 8 and aplurality of subcarriers are divided into sixteen RBs in one sub-frame.However, with the present invention, the number of sub-frames, of whichone frame is comprised, is not limited to eight and the number of RBsinto which a plurality of subcarriers are divided in one sub-frame isnot limited to sixteen.

Embodiments of the present invention have been described so far.

According to the present invention, transmission power of cell-specificRSs used only for LTE+ terminals (R4 to R7) among cell-specific RSs (R0to R7) may be smaller than transmission power of cell-specific RSs (R0to R3) used for both LTE terminals and LTE+ terminals. Terminals (LTEterminals and LTE+ terminals) from which the base station receivestransmission signals using four antennas are assumed to be located inthe entire cell. On the other hand, LTE+ terminals from which the basestation receives high-speed transmission signals using eight antennasare assumed to be located near the center of the cell where channelquality is good. For this reason, the base station transmitscell-specific RSs used only for LTE+ terminals (R4 to R7) with smallertransmission power than transmission power of cell-specific RSs (R0 toR3) used for both LTE terminals and LTE+ terminals, and can therebyimprove transmission efficiency of RSs. Furthermore, according to thepresent invention, the number of RS symbols per RB of cell-specific RSsused only for LTE+ terminals (R4 to R7) among cell-specific RSs (R0 toR7) (that is, RS mapping density) may be lower than the mapping densityof cell-specific RSs (R0 to R3) used for both LTE terminals and LTE+terminals.

The above embodiments have described a communication system in which LTEterminals and LTE+ terminals coexist. However, the present invention isapplicable not only to a communication system in which LTE terminals andLTE+ terminals coexist but is also applicable to a communication systemin which terminals corresponding to only a base station provided with,for example, N antennas and terminals corresponding to a base stationprovided with more than N antennas coexist. Furthermore, the presentinvention is also applicable to a case where terminal 1 operating, forexample, on communication system A, and terminal 2 operating only oncommunication system B which is an older version than communicationsystem A on which terminal 1 is operating coexist.

Furthermore, a case has been described in the above embodiments where R0to R3 are RSs transmitted from antennas 0 to 3 (first to fourthantennas) provided for a 4Tx base station or 8Tx base stations and R4 toR7 are RSs transmitted from antennas 4 to 7 (fifth to eighth antennas)provided for an 8Tx base station. However, R0 to R3 with the presentinvention have only to be RSs received by LTE terminals and LTE+terminals and R4 to R7 need only to be RSs received only by LTE+terminals. For example, R4 to R7 may be RSs transmitted to an LTE+terminal from another base station that performs coordinatedtransmission or a relay station.

Furthermore, the present invention is also applicable to a case wherethe number of antennas is equal to or more than five and less than eightin an 8Tx base station provided eight antennas—that is, when only partof RSs, R4 to R7, are transmitted as in the case of the aboveembodiments.

Furthermore, a case has been described in the above embodiments wherecell-specific RSs used only for LTE+ terminals are mapped to RBs shiftedin the frequency domain every sub-frame. However, according to thepresent invention, cell-specific RSs used only for LTE+ terminals mayalso be mapped to RBs not shifted in the frequency domain everysub-frame—that is, may be mapped to fixed RBs in any sub-frame.

Furthermore, a case has been described in the above embodiments wherethe base station does not allocate LTE terminals to RBs to whichcell-specific RSs used only for LTE+ terminals are mapped. Here, whenthe base station allocates LTE terminals to RBs to which cell-specificRSs used only for LTE+ terminals are mapped, LTE terminals receivecell-specific RSs used only for LTE+ terminals as data directed to theLTE terminals and the reception performance deteriorates. However,according to the present invention, when the performance deteriorationwith respect to the LTE terminals is in an allowable range, the basestation may allocate LTE terminals to RBs to which cell-specific RSsused only for LTE+ terminals are mapped.

Furthermore, the terminal may also be referred to as “UE,” the basestation may also be referred to as “Node B,” and the subcarrier may alsobe referred to as “tone.” Furthermore, a CP may also be referred to as“guard interval (GI)”. Furthermore, cell-specific RSs may also bereferred to as “common RSs.” Furthermore, a reference signal may also bereferred to as “pilot signal.” Furthermore, a sub-frame may also bereferred to as “slot.”

Furthermore, an antenna may also be referred to as “antenna port.” Aplurality of physical antennas may be used as one antenna port. An“antenna port” means a theoretical antenna comprised of one or aplurality of physical antennas. That is, the antenna port does notnecessarily refer to one physical antenna but may refer to an arrayantenna comprised of a plurality of antennas. For example, 3GPP-LTE doesnot define the number of physical antennas of which the antenna port iscomprised, but defines the antenna port as a minimum unit whereby thebase station can transmit different reference signals. Furthermore, theantenna port may be defined as a minimum unit whereby a precoding vectorweight is multiplied. For example, a base station provided with eightphysical antennas (physical antennas 0 to 7) transmits R0 with a weight(e.g. weighting factor (1, 1)) assigned thereto at physical antennas 0and 4 and transmits R4 with a weight orthogonal to the weight of R0(e.g. weighting factor (1, −1)) assigned thereto. Similarly, physicalantennas 1 and 5 transmit R1 with a weight (e.g. weighting factor (1,1)) assigned thereto and transmit R5 with a weight orthogonal to theweight of R1 (e.g. weighting factor (1, −1)) assigned thereto.Furthermore, physical antennas 2 and 6 transmit R2 with a weight (e.g.weighting factor (1, 1)) assigned thereto and transmit R6 with a weightorthogonal to the weight of R2 (e.g. weighting factor (1, −1)) assignedthereto. Furthermore, physical antennas 3 and 7 transmit R3 with aweight (e.g. weighting factor (1, 1)) assigned thereto and transmit R7with a weight orthogonal to the weight of R1 (e.g. weighting factor (1,−1)) assigned thereto. This allows LTE+ terminals to separate respectivepropagation paths from physical antennas 0 and 4 to the LTE+ terminalsusing R0 and R4 and perform channel estimation. Similarly, the LTE+terminals can separate respective propagation paths from physicalantennas 1 and 5 to the LTE+ terminals using R1 and R5 and performchannel estimation, separate respective propagation paths from physicalantennas 2 and 6 to the LTE+ terminals using R2 and R6 and separaterespective propagation paths from physical antennas 3 and 7 to the LTE+terminals using R3 and R7 and perform channel estimation. That is, thebase station transmits two cell-specific RSs with weights orthogonal toeach other assigned thereto from two physical antennas. Using such an RStransmission method, the present invention can also provide similareffects to those of the above embodiments.

Although a case has been described in the above embodiments where LTE+terminals use high-order MIMO (8-antenna MIMO), the present invention isnot limited to this, but the present invention is also applicable to acase where the receiving side (LTE+ terminal) receives reference signalsfor more antennas than those of 3GPP-LTE, for example, an operation ofreceiving reference signals transmitted form a plurality of basestations. For example, one base station comprises eight antennas in theabove embodiments, whereas the present invention is also applicable to aconfiguration in which a plurality of base stations configure eightantennas. The above embodiments have described 3GPP-LTE as having fourantennas and high-order MIMO as having a total of eight antennas withfour antennas further added to 3GPP-LTE as an example. However, thepresent invention is not limited to this, but 3GPP-LTE may have twoantennas and high-order MIMO may have a total of four antennas with twoantennas further added to 3GPP-LTE. Alternatively, the above two may becombined; 3GPP-LTE may be configured from two antennas or four antennasand high-order MIMO may be configured from two antennas or four antennasadded to 3GPP-LTE. Alternatively, 3GPP-LTE may be configured from twoantennas and high-order MIMO may be configured from a total of eightantennas with six antennas further added to 3GPP-LTE.

Furthermore, when the concept of an antenna port is used, even if thereare actually eight physical antennas, four antenna ports may be definedfor cell-specific RSs (cell-specific RSs used for both LTE terminals andLTE+ terminals) directed to 3GPP-LTE and other eight antenna ports maybe defined for cell-specific RSs (cell-specific RSs used only for LTE+terminals) directed to high-order MIMO. In this case, for example, thebase station may transmit cell-specific RSs directed to 3GPP-LTE withweights assigned thereto by two physical antennas per antenna port andtransmit cell-specific RSs directed to high-order MIMO without weightingfrom each antenna.

Furthermore, cell-specific RSs may also be defined as RSs used todemodulate broadcast information (PBCH) or PDCCH of the cell, andterminal-specific RSs may also be defined as RSs used to demodulatetransmission data directed to terminals.

Furthermore, the method of realizing conversion between the frequencydomain and time domain is not limited to IFFT or FFT.

Furthermore, the present invention is applicable not only to the basestation and terminals but also to all radio communication apparatuses.

The present invention has been described as an antenna in the aboveembodiments, but the present invention is likewise applicable to anantenna port.

The antenna port refers to a theoretical antenna comprised of one or aplurality of physical antennas. That is, the antenna port does notnecessarily refer to one physical antenna, but may refer to an arrayantenna comprised of a plurality of antennas.

For example, 3GPP-LTE does not define the number of physical antennas ofwhich the antenna port is comprised, but defines the antenna port as aminimum unit whereby the base station can transmit different referencesignals.

Furthermore, the antenna port may be defined as a minimum unit whereby aprecoding vector weight is multiplied.

Furthermore, CQI and PMI may be jointly referred to as “CSI (ChannelState Information).” Cell-specific RSs used only for LTE+ terminals inthe above embodiments are intended to measure CQI and PMI, and maytherefore be called “CSI-RS.”

Moreover, although cases have been described with the embodiments abovewhere the present invention is configured by hardware, the presentinvention may be implemented by software.

Each function block employed in the description of the aforementionedembodiment may typically be implemented as an LSI constituted by anintegrated circuit. These may be individual chips or partially ortotally contained on a single chip. “LSI” is adopted here but this mayalso be referred to as “IC,” “system LSI,” “super LSI” or “ultra LSI”depending on differing extents of integration.

Further, the method of circuit integration is not limited to LSI's, andimplementation using dedicated circuitry or general purpose processorsis also possible. After LSI manufacture, utilization of an FPGA (FieldProgrammable Gate Array) or a reconfigurable processor where connectionsand settings of circuit cells within an LSI can be reconfigured is alsopossible.

Further, if integrated circuit technology comes out to replace LSI's asa result of the advancement of semiconductor technology or a derivativeother technology, it is naturally also possible to carry out functionblock integration using this technology. Application of biotechnology isalso possible.

The disclosure of Japanese Patent Application No. 2009-018284, filed onJan. 29, 2009, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a mobile communication system orthe like.

1. A communication apparatus comprising: circuitry, which, in operation,maps a first reference signal in a subframe, and maps a second referencesignal in all downlink subframes, the first reference signal being usedfor computing a channel quality indicator (CQI) by a terminal compliantwith a first communication system, and the second reference signal beingused for computing a CQI by the terminal and another terminal compliantwith a second communication system; and a transmitter, which, inoperation, transmits the first reference signal to the terminal, andtransmits the second reference signal to the terminal and the otherterminal, wherein the first reference signal is mapped in a same periodas a period of semi-persistent scheduling (SPS) transmission, or in aperiod which is 1/N of the period of the SPS transmission, where N is apositive integer.
 2. The communication apparatus according to claim 1,wherein said circuitry, in operation, maps a third reference signal fordemodulating data on a resource block upon which the data is mapped, andsaid transmitter, in operation, transmits the third reference signal tothe terminal.
 3. The communication apparatus according to claim 2,wherein the third reference signal is a UE-specific reference signal. 4.The communication apparatus according to claim 1, wherein the secondreference signal is a cell-specific reference signal.
 5. Thecommunication apparatus according to claim 1, wherein the secondreference signal is used for demodulating a Physical Broadcast Channel(PBCH) or a downlink control channel.
 6. The communication apparatusaccording to claim 1, wherein the circuitry, in operation, maps thefirst reference signal and the second reference signal such that anumber of symbols of the first reference signal per resource block isless than a number of symbols of the second reference signal perresource block.
 7. The communication apparatus according to claim 1,wherein a maximum number of antenna ports of a base station compliantwith the first communication system is greater than a maximum number ofantenna ports of a base station compliant with the second communicationsystem.
 8. The communication apparatus according to claim 1, wherein thefirst communication system is an LTE-Advanced system, and the secondcommunication system is an LTE system.
 9. The communication apparatusaccording to claim 1, wherein the circuitry, in operation, maps thefirst reference signal in a set period.
 10. The communication apparatusaccording to claim 1, wherein the circuitry, in operation, maps thefirst reference signal in a same period as a period in which the CQI isreported, or in a period which is an integer multiple of the period inwhich the CQI is reported.
 11. The communication apparatus according toclaim 1, wherein the first reference signal is a cell-specific referencesignal.
 12. The communication apparatus according to claim 1, whereinthe first reference signal is a channel state information-referencesignal (CSI-RS).
 13. A communication method comprising: mapping a firstreference signal in a subframe; mapping a second reference signal in alldownlink subframes, the first reference signal being used for computinga channel quality indicator (CQI) by a terminal compliant with a firstcommunication system, and the second reference signal being used forcomputing a CQI by the terminal and another terminal compliant with asecond communication system; and transmitting the first reference signalto the terminal, and transmitting the second reference signal to theterminal and the other terminal, wherein the first reference signal ismapped in a same period as a period of semi-persistent scheduling (SPS)transmission, or in a period which is 1/N of the period of the SPStransmission, where N is a positive integer.